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Abstract:

The present invention provides a nonaqueous electrolyte secondary
battery, comprising an electrode group including a positive electrode, a
negative electrode including a material for absorbing-desorbing lithium
ions, and a separator arranged between the positive electrode and the
negative electrode, a nonaqueous electrolyte impregnated in the electrode
group and including a nonaqueous solvent and a lithium salt dissolved in
the nonaqueous solvent, and a jacket for housing the electrode group and
having a thickness of 0.3 mm or less, wherein the nonaqueous solvent
γ-butyrolactone in an amount larger than 50% by volume and not
larger than 95% by volume based on the total amount of the nonaqueous
solvent.

Claims:

1. A nonaqueous electrolyte secondary battery, comprising: an electrode
group including a positive electrode, a negative electrode containing a
material for absorbing-desorbing lithium ions, and a separator arranged
between the positive electrode and the negative electrode; a nonaqueous
electrolyte impregnated in said electrode group and including a
nonaqueous solvent and a lithium salt dissolved in said nonaqueous
solvent; and a jacket for housing said electrode group, said jacket being
made of a sheet having a thickness of 0.05 mm to 0.35 mm, wherein said
nonaqueous solvent contains γ-butyrolactone in an amount of 40 to
95% by volume based on the total amount of the nonaqueous solvent.

2-30. (canceled)

31. The battery according to claim 1, wherein the nonaqueous solvent
contains 60 to 95% by volume of γ-butyrolactone.

32. The battery according to claim 1, wherein the nonaqueous solvent
further includes ethylene carbonate.

33. The battery according to claim 32, wherein the nonaqueous solvent
further includes at least one kind of a third solvent selected from the
group consisting of propylene carbonate, vinylene carbonate,
trifluoropropylene, diethyl carbonate, methyl ethyl carbonate and an
aromatic compound.

34. The battery according to claim 1, wherein the positive electrode and
the separator are adhered to each other by an adhesive polymer present in
at least a part of the boundary therebetween, and the negative electrode
and the separator are adhered to each other by an adhesive polymer
present in at least a part of the boundary therebetween.

35. The battery according to claim 1, wherein each of the positive
electrode and the negative electrode further contains a binder, and these
positive electrode and negative electrode and the separator are made
integral by the thermal curing of the binder.

36. The battery according to claim 1, wherein the separator includes a
porous sheet having an air permeability of 600 seconds/100 cm3 or
less.

37. The battery according to claim 1, wherein the material for
absorbing-desorbing lithium ions contains at least one kind of a
carbonaceous material and a graphitized material selected from the group
consisting of graphite, coke, carbon fibers and spherical carbon.

38. The battery according to claim 37, wherein the carbon fibers are
mesophase pitch-based carbon fibers.

39. The battery according to claim 1, wherein the material for
absorbing-desorbing lithium ions comprises a metal oxide.

40. The battery according to claim 10, wherein the metal oxide comprises
a lithium titanium oxide.

41. The battery according to claim 1, wherein the sheet of the jacket
comprises a resin.

42. The battery according to claim 41, wherein the resin comprises at
least one kind of a polymer selected from the group consisting of
polyethylene and polypropylene.

43. The battery according to claim 42, wherein the resin comprises a
heat-fusible resin.

44. The battery according to claim 1, wherein the sheet of the jacket
comprises at least one kind of a metal selected from the group consisting
of aluminum, stainless steel, iron, copper or nickel.

45. The battery according to claim 1, wherein the sheet of the jacket
comprises a metal layer and a flexible synthetic resin layer formed on at
least a portion of the metal layer.

Description:

[0002] Recently, a lithium ion secondary battery has been put on the
market as a nonaqueous electrolyte secondary battery for portable
apparatuses such as portable phones. This battery uses lithium cobalt
oxide (e.g., LiCoO2) as a positive electrode active material, a
graphite material or carbonaceous material as a negative electrode active
material, an organic solvent having a lithium salt dissolved as a
nonaqueous electrolyte, and a porous film as a separator. A nonaqueous
solvent having a low viscosity and a low boiling point is used as a
solvent of the electrolyte. For example, Japanese Patent Disclosure
(Kokai) No. 4-14769 discloses a nonaqueous electrolyte secondary battery
comprising an electrolyte including a mixed solvent consisting
essentially of propylene carbonate, ethylene carbonate and
γ-butyrolactone, the γ-butyrolactone occupying 10 to 50% by
volume of the entire solvent mixture. On the other hand, Japanese Patent
Disclosure (Kokai) No. 11-97062 discloses a nonaqueous electrolyte
secondary battery using a nonaqueous electrolyte prepared by dissolving
lithium borofluoride (LiBF4) in a solvent consisting of 100% by
volume of γ-butyrolactone.

[0003] It is of high importance nowadays to decrease the thickness of the
secondary battery in accordance with decrease in the thickness of the
portable apparatus. In order to decrease the thickness of the secondary
battery, it is necessary to decrease the thickness of the jacket housing
the positive electrode, the negative electrode, the separator, and the
nonaqueous electrolyte. However, in the nonaqueous secondary battery
provided with a nonaqueous electrolyte including a mixed solvent
containing 10 to 50% by volume of γ-butyrolactone, a reaction takes
place between the positive electrode and the nonaqueous electrolyte when
the secondary battery is stored at such a high temperature as 60°
C. or higher. As a result, the nonaqueous electrolyte is decomposed by
oxidation to generate a gaseous material. And also, a gas material is
likely to be generated from the negative electrode during the initial
charging. What should be noted is that, if the thickness of the jacket is
decreased, the jacket is swollen by the gas generation so as to be
deformed. If the jacket is deformed, an electronic equipment cannot be
housed in the battery. Alternatively, malfunction of the electronic
equipment tends to be invited.

[0004] In the nonaqueous electrolyte secondary battery, it is also
important to further improve the large discharge characteristics and the
charge-discharge cycle characteristics.

[0005] An object of the present invention is to provide a nonaqueous
electrolyte secondary battery that permits suppressing the gas generation
during storage of the battery under high temperatures so as to prevent a
jacket from being swollen and also permits improving the large discharge
characteristics and the charge-discharge cycle life.

[0006] According to the present invention, there is provided a nonaqueous
electrolyte secondary battery, comprising an electrode group including a
positive electrode, a negative electrode containing a material for
absorbing-desorbing lithium ions, and a separator arranged between the
positive electrode and the negative electrode, a nonaqueous electrolyte
impregnated in the electrode group and including a nonaqueous solvent and
a lithium salt dissolved in the nonaqueous solvent, and a jacket for
housing the electrode group and having a thickness of 0.3 mm or less,
wherein the nonaqueous solvent contains γ-butyrolactone in an
amount larger than 50% by volume and not larger than 95% by volume based
on the total amount of the nonaqueous solvent.

[0007] According to the present invention, there is provided a nonaqueous
electrolyte secondary battery, comprising an electrode group including a
positive electrode, a negative electrode containing a material for
absorbing-desorbing lithium ions, and a separator arranged between the
positive electrode and the negative electrode, a nonaqueous electrolyte
impregnated in the electrode group and including a nonaqueous solvent and
a lithium salt dissolved in the nonaqueous solvent, and a jacket for
housing the electrode group, the jacket being made of a sheet having a
thickness of 0.5 mm or less including a resin layer, wherein the
nonaqueous solvent contains γ-butyrolactone in an amount larger
than 50% by volume and not larger than 95% by volume based on the total
amount of the nonaqueous solvent.

[0008] Further, according to the present invention, there is provided a
nonaqueous electrolyte secondary battery, comprising an electrode group
including a positive electrode including a collector and a positive
electrode layer formed on one or both surfaces of the collector and
containing an active material, a negative electrode including a collector
and a negative electrode layer formed on one or both surfaces of the
collector and containing a material for absorbing-desorbing lithium ions,
and a separator arranged between the positive electrode and the negative
electrode, a nonaqueous electrolyte impregnated in the electrode group
and including a nonaqueous solvent and a lithium salt dissolved in the
nonaqueous solvent, and a jacket for housing the electrode group and
having a thickness of 0.3 mm or less, wherein the positive electrode
layer has a porosity lower than that of the negative electrode layer, the
positive electrode layer has a thickness of 10 to 100 μm, and the
nonaqueous solvent contains 40 to 95% by volume of γ-butyrolactone
based on the total amount of the nonaqueous solvent.

[0009] Further, according to the present invention, there is provided a
nonaqueous electrolyte secondary battery, comprising an electrode group
including a positive electrode including a collector and a positive
electrode layer formed on one or both surfaces of the collector and
containing an active material, a negative electrode including a collector
and a negative electrode layer formed on one or both surfaces of the
collector and containing a material for absorbing-desorbing lithium ions,
and a separator arranged between the positive electrode and the negative
electrode, a nonaqueous electrolyte impregnated in the electrode group
and including a nonaqueous solvent and a lithium salt dissolved in the
nonaqueous solvent, and a jacket for housing the electrode group, the
jacket being made of a sheet having a thickness of 0.5 mm or less
including a resin layer, wherein the positive electrode layer has a
porosity lower than that of the negative electrode layer, the positive
electrode layer has a thickness of 10 to 100 μm, and the nonaqueous
solvent contains 40 to 95% by volume of γ-butyrolactone based on
the total amount of the nonaqueous solvent.

[0010] Additional objects and advantages of the invention will be set
forth in the description which follows, and in part will be obvious from
the description, or may be learned by practice of the invention. The
objects and advantages of the invention may be realized and obtained by
means of the instrumentalities and combinations particularly pointed out
hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0011] The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate presently preferred embodiments
of the invention, and together with the general description given above
and the detailed description of the preferred embodiments given below,
serve to explain the principles of the invention.

[0012] FIG. 1 is a cross sectional view exemplifying a nonaqueous
electrolyte secondary battery according to first embodiment of the
present invention;

[0013]FIG. 2 is a cross sectional showing in a magnified fashion a
portion A shown in FIG. 1;

[0014] FIG. 3 schematically shows the boundary regions between the
positive electrode and the separator and between the separator and the
negative electrode in the secondary battery shown in FIG. 1;

[0015] FIG. 4 is a cross sectional view for explaining the thickness of a
positive electrode active material layer in a nonaqueous electrolyte
secondary battery according to a second embodiment of the present
invention;

[0016] FIG. 5 is a cross sectional view exemplifying a nonaqueous
electrolyte secondary battery according to a second embodiment of the
present invention;

[0017] FIG. 6 is a cross sectional view showing in a magnified fashion a
portion B shown in FIG. 5; and

[0018] FIG. 7 schematically shows the boundary regions between the
positive electrode and the separator and between the separator and the
negative electrode in the secondary battery shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

[0019] A nonaqueous electrolyte secondary battery according to a first
embodiment of the present invention comprises an electrode group having a
positive electrode, a negative electrode for absorbing and desorbing
lithium ions, and a separator interposed between the positive electrode
and the negative electrode, a nonaqueous electrolyte permeating the
electrode group to have the electrode group impregnated with the
nonaqueous electrolyte and containing a nonaqueous solvent and a lithium
salt dissolved in the nonaqueous solvent, and a jacket housing the
electrode group. The nonaqueous solvent contains γ-butyrolactone in
an amount larger than 50% by volume and not larger than 95% by volume
based on the total amount of the nonaqueous solvent.

[0020] In the secondary battery of the present invention, it is possible
for the positive electrode, the negative electrode and the separator not
to be formed as an integral structure. However, it is desirable to form
an integral structure under the conditions given in item (a) or (b):

[0021] (a) The positive electrode and the separator are adhered to each
other by an adhesive polymer present in at least a part of the boundary
therebetween, and the negative electrode and the separator are adhered to
each other by an adhesive polymer present in at least a part of a
boundary therebetween. Particularly, the positive electrode and the
separator preferably be adhered to each other by an adhesive polymer
dotted inside and at the boundary between the positive electrode and the
separator. Also, the negative electrode and the separator preferably be
adhered to each other by an adhesive polymer dotted inside and at the
boundary between the negative electrode and the separator.

[0022] (b) The positive electrode, the negative electrode and the
separator are made integral by thermally curing a binder contained in
each of the positive electrode and the negative electrode.

[0023] The construction in any of items (a) and (b) given above makes it
possible to further suppress the swelling of the jacket.

[0024] It is desirable for the secondary battery of the present invention
to have a product between the battery capacity (Ah) and the battery
internal impedance (mΩ) at 1 kHz of not smaller than 10 mΩAh
and not larger than 110 mΩAh. Incidentally, the battery capacity
denotes a nominal capacity or the discharge capacity at the time when the
battery is discharged at a discharge rate of 0.2 C. More preferably, the
product between the battery capacity and the battery internal impedance
at 1 kHz should fall within a range of between 20 mΩAh and 60
mΩAh.

[0025] The product between the battery capacity and the battery internal
impedance at 1 kHz can be set to fall within a range of between 10
mΩAh and 110 mΩAh by the "Manufacturing method (I)" or
"Manufacturing method (II)" described later. In the Manufacturing method
(I), the addition amount and distribution of the adhesive polymer and the
initial charging conditions are set to permit the product between the
battery capacity and the internal impedance should be set to fall within
a range of between 10 mΩAh and 110 mΩAh. In the Manufacturing
method (II), the temperature and the pressure in the pressing step for
forming the electrode group and the initial charging conditions should be
set to permit the product between the battery capacity and the internal
impedance to fall within a range of between 10 mΩAh and 110
mΩAh.

[0026] The nonaqueous electrolyte secondary battery equipped with an
electrode group meeting the conditions given in item (a) above will now
be described.

[0027] 1) Positive Electrode

[0028] This positive electrode has a structure in which an electrode layer
containing an active material is carried by one or both surfaces of the
collector. The positive electrode holds adhesive polymers in voids. The
positive electrode layer preferably further contains a conducting agent
and a binder.

[0030] Examples of this conducting agent are one or more types of carbon
materials selected from acetylene black, carbon black, and graphite.

[0031] The binder has functions of allowing the collector to hold the
active material in place and binding particles of the active material to
each other. As this binder, it is possible to use one or more types of
polymers selected from polytetrafluoroethylene (PTA),
ethylene-propylene-diene copolymer (EPODE), styrene-butadiene rubber
(SBR), and polyvinylidene fluoride (PVdF).

[0032] It is desirable to set the mixing amount of the positive electrode
active material at 80 to 95% by weight, the mixing amount of the
conducting agent at 3 to 20% by weight, and the binder at 2 to 7% by
weight.

[0033] As the collector, a porous conductive substrate or a conductive
substrate having no pores can be used. These conductive substrates can be
made from, e.g., aluminum, stainless steel, or nickel.

[0034] As this collector, it is particularly preferable to use a
conductive substrate having a two-dimensional porous structure in which
pores 3 mm or less in diameter exist at a ratio of one or more pores per
10 cm2. That is, if the diameter of the pores formed in the
conductive substrate is larger than 3 mm, no satisfactory positive
electrode strength may be obtained. On the other hand, if the ratio of
pores 3 mm or less in diameter is smaller than the above range, it
becomes difficult to allow the nonaqueous electrolyte to uniformly
permeate into the electrode group, so no satisfactory cycle life may be
obtained. The pore diameter is preferably 0.1 to 1 mm. The pore ratio is
preferably 10 to 20 pores per 10 cm2.

[0035] The conductive substrate having a two-dimensional porous structure
in which pores 3 mm or less in diameter exist at a ratio of one or more
pores per 10 cm2 preferably has a thickness of 15 to 100 μm. If
the thickness is less than 15 μm, no satisfactory positive electrode
strength may be obtained. If the thickness exceeds 100 μm, the battery
weight and the electrode group thickness increase. This may make it
difficult to well increase the weight energy density and volume energy
density of the thin secondary battery. A more favorable range of the
thickness is 30 to 80 μm.

[0036] 2) Negative Electrode

[0037] The negative electrode has a structure in which an electrode layer
containing an active material is carried by one or both surfaces of the
collector. The negative electrode holds adhesive polymers in voids. The
negative electrode layer preferably further contains a binder.

[0038] An example of the active material is a carbon material which
absorbs and desorbs lithium ions. Examples of this carbon material are a
graphitized material and carbonaceous material such as graphite, coke,
carbon fibers, and spherical carbon, and a graphitized material and
carbonaceous material obtained by heat-treating a thermosetting resin,
isotropic pitch, mesophase pitch, mesophase pitch-based carbon fibers,
and mesophase globules (mesophase pitch-based carbon fibers are
particularly preferred because the battery capacity and the
charge-discharge cycle characteristics can be improved) at 500 to
3,000° C. Of these materials, it is preferable to use a
graphitized material obtained by heat-treating at 2,000° C. or
more and having a graphite crystal in which an interplanar spacing
d002 derived from (002) reflection is 0.340 nm or less. It is
possible to greatly improve the battery capacity and large discharge
characteristic of a nonaqueous electrolyte secondary battery including a
negative electrode containing this graphitized material as a carbon
material. The interplanar spacing d002 is more preferably 0.336 nm
or less.

[0040] It is desirable to-set the mixing amount of the carbon material at
90 to 98% by weight, the mixing amount of the binder at 2 to 20% by
weight.

[0041] As the collector, a porous conductive substrate or a conductive
substrate having no pores can be used. These conductive substrates can be
made from, e.g., copper, stainless steel, or nickel.

[0042] As this collector, it is particularly preferable to use a
conductive substrate having a two-dimensional porous structure in which
pores 3 mm or less in diameter exist at a ratio of one or more pores per
10 cm2. That is, if the diameter of the pores formed in the
conductive substrate is larger than 3 mm, no satisfactory negative
electrode strength may be obtained. On the other hand, if the ratio of
pores 3 mm or less in diameter is smaller than the above range, it
becomes difficult to allow the nonaqueous electrolyte to uniformly
permeate into the electrode group, so no satisfactory cycle life may be
obtained. The pore diameter is preferably 0.1 to 1 mm. The pore ratio is
preferably 10 to 20 pores per 10 cm2.

[0043] The conductive substrate having a two-dimensional porous structure
in which pores 3 mm or less in diameter exist at a ratio of one or more
pores per 10 cm2 preferably has a thickness of 10 to 50 μm. If
the thickness is less than 10 μm, no satisfactory negative electrode
strength may be obtained. If the thickness exceeds 50 μm; the battery
weight and the electrode group thickness increase. This may make it
difficult to well increase the weight energy density and volume energy
density of the thin secondary battery.

[0044] The negative electrode layer contains a carbon material capable of
absorbing-desorbing lithium ions as described previously. In addition, it
is possible for the negative electrode layer to contain metals such as
aluminum, magnesium, tin, and silicon, a metal compound selected from the
group consisting of a metal oxide, a metal sulfide, and a metal nitride,
and a lithium alloy.

[0051] As this porous sheet, for example, a porous film or a nonwoven
fabric can be used. The porous sheet is preferably made from at-least one
type of material selected from, e.g., polyolefin and cellulose. Examples
of polyolefin are polyethylene and polypropylene. Of these materials, a
porous film made from one or both of polyethylene and polypropylene is
preferred because the safety of the secondary battery can be improved.

[0052] The thickness of the porous sheet is preferably 30 μm or less.
If the thickness exceeds 30 μm, the distance between the positive and
negative electrodes increases, and this may increase the internal
resistance. The lower limit of the thickness is preferably 5 μm. If
the thickness is less than 5 μm, the separator strength may
significantly lower to allow easy internal short circuit. The upper limit
of the thickness is more preferably 25 μm, and its lower limit is more
preferably 10 μm.

[0053] The heat shrinkage ratio of the porous sheet upon being left to
stand at 120° C. for 1 hr is preferably 20% or less. If this heat
shrinkage ratio exceeds 20%, it may become difficult to obtain
satisfactory adhesion strength between the positive and negative
electrodes and the separator. The heat shrinkage ratio is more preferably
15% or less.

[0054] The porous sheet preferably has a porosity of 30 to 60% for the
reasons explained below. If the porosity is less than 30%, good
electrolyte holding properties may become difficult to obtain in the
separator. On the other hand, if the porosity exceeds 60%, no
satisfactory separator strength may be obtained. A more favorable range
of the porosity is. 35 to 50%.

[0055] The air permeability of the porous sheet is preferably 600 sec/100
cm3 or less. The term "air permeability" represents the time
(second) required for 100 cm3 of air to permeate through a porous
sheet. If the air permeability exceeds 600 sec/100 cm3, high lithium
ion mobility may become difficult to obtain in the separator. The lower
limit of this air permeability is preferably 100 sec/100 cm3. If the
air permeability is less than 100 sec/100 cm3, no satisfactory
separator strength may be obtained. The upper limit of the air
permeability is more preferably 500 sec/100 cm3, and most preferably
400 sec/100 cm3. The lower limit of the air permeability is more
preferably 150 sec/100 cm3.

[0056] 4) Nonaqueous Electrolyte

[0057] The nonaqueous electrolyte used in the present invention is
prepared by dissolving a lithium salt in a mixed nonaqueous solvent
containing γ-butyrolactone (BL) as a main component. The BL content
of the mixed nonaqueous solvent should fall within a range of larger than
50% by volume and not larger than 95% by volume. If the BL content is not
more than 50% by volume, a gas is likely to be generated under high
temperatures. Also, where the mixed nonaqueous solvent contains BL and a
cyclic carbonate, the ratio of the cyclic carbonate is rendered
relatively high, leading to a high viscosity of the solvent and to a low
conductivity of the nonaqueous electrolyte. As a result, the
charge-discharge cycle characteristics, the large discharge
characteristics and the discharge characteristics under low temperatures,
e.g., about -20° C., are deteriorated. On the other hand, if the
BL content exceeds 95% by weight, a reaction takes place between the
negative electrode and BL so as to impair the charge-discharge cycle
characteristics. To be more specific, if the nonaqueous electrolyte is
reduced and decomposed as a result of the reaction between BL and the
negative electrode containing, for example, a carbon material
absorbing-desorbing lithium ions, a film inhibiting the charge-discharge
reaction is formed on the surface of the negative electrode. As a result,
a current concentration tends to be generated in the negative electrode
so as to bring about problems. For example, lithium metal is precipitated
on the surface of the negative electrode. Also, the impedance at the
interface of the negative electrode is increased so as to lower the
charge-discharge efficiency of the negative electrode and, thus, to
impair the charge-discharge cycle characteristics. Preferably, the BL
content of the nonaqueous solvent should fall within a range of between
60% by volume and 95% by volume. Where the BL content falls within the
range noted above, the gas generation during storage of the secondary
battery under high temperatures can be suppressed more effectively. Also,
it is possible to further improve the discharge capacity under low
temperatures about -20° C. More preferably, the BL content of the
nonaqueous solvent should fall within a range of between 65% by volume
and 90% by volume.

[0058] It is desirable to use a cyclic carbonate together with BL in the
present invention because the cyclic carbonate permits improving the
charge-discharge efficiency.

[0059] The cyclic carbonate used in the present invention includes, for
example, propylene carbonate (PC), ethylene carbonate (EC), vinylene
carbonate (VC), and trifluoropropylene carbonate (TFPC). Particularly, if
EC is used together with BL, the charge-discharge characteristics and the
large discharge characteristics can be markedly improved. It is also
desirable to prepare a mixed solvent by mixing BL with at least one kind
of a third solvent selected from the group consisting of PC, VC, TFPC,
diethyl carbonate (DEC), methyl ethyl carbonate (MEC) and an aromatic
compound. The mixed solvent of the particular construction permits
improving the charge-discharge cycle characteristics.

[0060] In order to decrease the viscosity of the mixed solvent, it is
possible for the nonaqueous solvent containing BL to further contain 20%
by volume or less of a solvent having a low viscosity selected from the
group consisting of, for example, a chain carbonate, a chain ether, and a
cyclic ether.

[0061] Preferred combinations of the nonaqueous solvents used in the
present invention include, for example, a combination of BL and EC, a
combination of BL and PC, a combination of BL, EC and DEC, a combination
of BL, EC and MEC, a combination of BL, EC, MEC and VC, a combination of
BL, EC and VC, a combination of BL, PC and VC, and a combination of BL,
EC, PC and VC. In this case, it is desirable to set the mixing ratio of
EC to fall within a range of between 5 and 40% by volume. It should be
noted in this connection that, if the mixing amount of EC is smaller than
5% by volume, it is difficult to cover densely the surface of the
negative electrode with a protective film, giving rise to a possibility
that a reaction may take place between the negative electrode and BL. As
a result, it is difficult to improve sufficiently the charge-discharge
cycle characteristics. On the other hand, if the mixing amount of EC
exceeds 40% by volume, the viscosity of the nonaqueous electrolyte is
unduly increased so as to lower the ionic conductance. As a result, it is
difficult to improve sufficiently the charge-discharge cycle
characteristics, the large discharge characteristics, and the low
temperature discharge characteristics. More preferably, the EC amount
should fall within a range of between 10 and 35% by volume. Also, the
solvent consisting of at least one compound selected from the group
consisting of DEC, MEC, PC and VC serves to forms a dense protective film
on the surface of the negative electrode so as to lower the impedance at
the interface of the negative electrode. The addition amount of any of
these solvents is not particularly limited as far as it is possible to
obtain the particular function described above. It should be noted,
however, that, if the mixing ratio of at least one solvent selected from
the group consisting of DEC, MEC, PC and VC exceeds 10% by volume, it is
difficult to prevent sufficiently the nonaqueous electrolyte from being
decomposed by oxidation under high temperatures. Alternatively, the
viscosity of the nonaqueous electrolyte tends to be increased so as to
lower the ionic conductance. Therefore, it is desirable to use at least
one solvent selected from the group consisting of DEC, MEC, PC and VC in
an amount not exceeding 10% by volume. More preferably, at least one of
these solvents should be used in an amount of 2% by volume or less. On
the other hand, the lower limit in the addition amount of at least one of
these solvents should be 0.001% by volume, preferably, 0.05% by volume.

[0062] Particularly, it is desirable for the nonaqueous solvent to contain
BL, EC and VC. The BL content is larger than 50% by volume and not larger
than 95% by volume. The nonaqueous electrolyte secondary battery
comprising a nonaqueous electrolyte containing the nonaqueous solvent
noted above and a negative electrode containing a carbon material capable
of absorbing-desorbing lithium ions permits markedly lowering the
impedance at the interface of the negative electrode and also permits
suppressing the metal lithium precipitation on the negative electrode so
as to improve the charge-discharge efficiency of the negative electrode.
As a result, it is possible to suppress the gas generation during storage
of the secondary battery under high temperatures, thereby preventing a
jacket having a thickness of 3 mm or less from being deformed while
realizing an excellent large discharge characteristics and a long life.
It is considered reasonable to understand that the reasons for such a
prominent improvement of the negative electrode characteristics are as
follows. Specifically, in the secondary battery, a protective film
consisting of EC is formed on the surface of the negative electrode, and
a thin and dense film consisting of VC is further formed on the surface
of the negative electrode. It is considered reasonable to understand that
the reaction between BL and the negative electrode is inhibited, thereby
lowering the impedance and preventing the metal lithium precipitation.

[0063] It is possible to use a nonaqueous solvent containing BL in an
amount larger than 50% by volume and not larger than 95% by volume, EC
and an aromatic compound in place of the mixed nonaqueous solvent of the
composition described previously. The aromatic compound is at least one
compound selected from the group consisting of benzene, toluene, xylene,
biphenyl and terphenyl. EC is deposited on the surface of the negative
electrode containing, for example, a carbon material capable of
absorbing-desorbing lithium ions so as to form a protective film, thereby
suppressing the reaction between the negative electrode and BL. In this
case, it is desirable for the mixed nonaqueous solvent to contain 5 to
40% by volume of EC for the reasons described previously. Preferably, the
EC content should fall within a range of between 10% by volume and 35% by
volume. On the other hand, the benzene ring of the aromatic compound
tends to be adsorbed easily on the surface of the negative electrode
containing, for example, a carbon material capable of absorbing-desorbing
lithium ions so as to suppress the reaction between BL and the negative
electrode. As described above, the nonaqueous electrolyte based on a
mixed nonaqueous solvent containing BL in an amount larger than 50% by
volume and not larger than 95% by volume, EC and an aromatic compound
makes it possible to suppress sufficiently the reaction between the
negative electrode and BL so as to improve the charge-discharge cycle
characteristics of the secondary battery. It is desirable for the mixed
nonaqueous solvent to further contain at least one solvent selected from
the group consisting of DEC, MEC, PC, TFPC and VC. By adding at least one
solvent selected from the group consisting of DEC, MEC, PC, TFPC and VC,
the reaction between the negative electrode and BL can be suppressed more
effectively, leading to a further improvement in the charge-discharge
cycle characteristics. Particularly, it is desirable to use VC as an
additional solvent. The addition amount of a third solvent consisting of
at least one compound selected from the group consisting of an aromatic
compound, DEC, MEC, PC, TFPC and VC is not particularly limited. In other
words, the addition amount can be determined appropriately as far as the
particular function described above can be performed. It should be noted,
however, that, if the mixing ratio of the third solvent exceeds 10% by
volume, it is difficult to suppress sufficiently the decomposition of the
nonaqueous electrolyte by oxidation under high temperatures.
Alternatively, the viscosity of the nonaqueous electrolyte tends to be
increased so as to lower the ionic conductance. Naturally, it is
desirable for the volume ratio of the third solvent in the nonaqueous
solvent to be at most 10% by volume. Preferably, the volume ratio of the
third solvent should be at most 2% by volume. On the other hand, the
lower limit of the volume ratio of the third component should be 0.001%
by volume, preferably 0.05% by volume.

[0065] The amount of the electrolytic salt dissolved in the nonaqueous
solvent should desirably be 0.5 to 2.0 mol/l (liter).

[0066] The amount of nonaqueous electrolyte is preferably 0.2 to 0.6 g per
100 mAh of battery unit capacity for the reasons explained below. If the
nonaqueous electrolyte amount is less than 0.2 g/100 mAh, it may become
impossible to well maintain the ion conductivity of the positive and
negative electrodes. On the other hand, if the nonaqueous electrolyte
amount exceeds 0.6 g/100 mAh, this large electrolyte amount may make
sealing difficult when a film jacket is used. A more favorable range of
the nonaqueous electrolyte amount is 0.4 to 0.55 g/100 mAh.

[0067] 5) Adhesive Polymer

[0068] As the adhesive polymer, it is possible to use one or more types of
polymers selected from the group consisting of polyacrylonitrile (PAN),
polyacrylate (PMMA), polyvinylidene fluoride (PVdF), polyvinyl chloride
(PVC), and polyethylene oxide (PEO). It is desirable for this adhesive
polymer to be kept highly adhesive while this adhesive polymer holds the
nonaqueous electrolyte in place. More preferably, this adhesive polymer
should have high lithium ion conductivity. PAN, PMMA, PVdF, PVC and PEO
are examples of the polymer that is highly adhesive while holding the
nonaqueous electrode in place and has a high lithium ion conductivity. Of
these polymers, PVdF is most preferred. PVdF can hold the nonaqueous
electrolyte in place and partially gels in the presence of the nonaqueous
electrolyte. This further improves the ion conductivity of the positive
electrode.

[0069] It is desirable for the adhesive polymer to assume a porous
structure having fine pores within the cavities of the positive
electrode, the negative electrode, and the separator. The adhesive
polymer assuming a porous structure is capable of holding the nonaqueous
electrolyte.

[0070] The total amount of adhesive polymers contained in the battery is
preferably 0.1 to 6 mg per 100 mAh battery capacity for the reasons
explained below. If the total amount of adhesive polymers is less than
0.1 mg per 100 mAh battery capacity, it may become difficult to well
increase the adhesion strength between the positive electrode and the
separator and between the negative electrode and the separator. On the
other hand, if the total amount exceeds 6 mg per 100 mAh battery
capacity, the lithium ion conductivity of the secondary battery may
decrease, or its internal resistance may increase. This may make it
difficult to well improve the discharge capacity, large discharge
characteristic, and charge/discharge cycle life. A more preferable range
of the total amount of adhesive polymers is 0.2 to 1 mg per 100 mAh
battery capacity.

[0071] 6) Jacket

[0072] A first jacket made of a sheet having a thickness of 0.5 mm or less
including a resin layer or a second jacket having a thickness of 0.3 mm
or less is used in the present invention. Each of the first and second
jackets is light in weight, making it possible to increase the energy
density per unit weight of the battery. However, each of these jackets,
which are flexible, tend to be deformed by the gas generated from the
electrode group or from the nonaqueous electrolyte.

[0073] The resin layer included in the first jacket performs the function
of a protective layer and can be made of, for example, polyethylene or
polypropylene. To be more specific, the first jacket consists of a sheet
comprising a metal layer and protective layers formed on both surfaces of
the metal layer. The metal layer, which serves to shield water, can be
made of, for example, aluminum, stainless steel, iron, copper or nickel.
Particularly, it is desirable to use an aluminum layer that is light in
weight and high in its function of shielding water. The metal layer can
be formed of a single metal layer. Alternatively, a plurality of metal
layers can be bonded to each other to form the metal layer included in
the first jacket. The protective layer formed on the outer surface of the
metal layer serves to prevent a damage done to the metal layer. The outer
protective layer can be formed of a single resin layer. Alternatively, a
plurality of resin layers can be laminated one upon the other to form the
outer protective layer. On the other hand, the protective layer formed on
the inner surface of the metal layer serves to prevent the metal layer
from being corroded by the nonaqueous electrolyte. The inner protective
layer can be formed of a single resin layer. Alternatively, a plurality
of resin layers can be laminated one upon the other to form the inner
protective layer. It is also possible to form a heat-fusible resin (for
example thermoplastic adhesive) layer on the surface of the inner
protective layer.

[0074] If the thickness of the first jacket exceeds 0.5 mm, the capacity
per unit weight of the battery is lowered. The thickness of the first
jacket should desirably be 0.3 mm or less, more preferably 0.25 mm or
less, and most preferably 0.15 mm or less. On the other hand, the lower
limit of the thickness should desirably be set at 0.05 mm. If the
thickness is smaller than 0.05 mm, the first jacket tends to be deformed
and broken. The lower limit of the thickness should more desirably be
0.08 mm and most desirably be 0.1 mm.

[0075] As the second jacket, a metal can or a film having a function of
shielding water can be used. An example of the film is a laminate film
including a metal layer and a flexible synthetic resin layer formed on at
least a portion of the metal layer. Examples of the metal layer are
aluminum, stainless steel, iron, copper, and nickel. Of these metals,
aluminum that is light in weight and has a high function of shielding
water is preferred. Examples of the synthetic resin are polyethylene and
polypropylene.

[0076] If the second jacket is thicker than 0.3 mm, the secondary battery
cannot be made sufficiently thin, resulting in failure to obtain a
sufficiently high weight energy density. It is desirable for the second
jacket to have a thickness of 0.25 mm or less and, more preferably 0.15
mm or less. On the other hand, the lower limit in the thickness of the
second jacket should desirably be set at 0.05 mm. If the second jacket is
thinner than 0.05 mm, the second jacket tends to be deformed and broken.
The lower limit in the thickness of the second jacket should more
preferably be 0.08 mm and most desirably be 0.1 mm. Particularly, the
thickness of the second jacket should desirably fall within a range of
between 0.05 mm and 0.3 mm, and more preferably within a range of between
0.08 mm and 0.15 mm.

[0077] For measuring the thickness of the jacket, three optional points
apart from each other by at least 1 cm are selected within a region
excluding the sealing portion of the jacket, and the thickness in each of
these three points is actually measured. The measured values are averaged
to determine the thickness of the jacket. If a foreign matter such as a
resin is attached to the surface of the jacket, the thickness is measured
after removal of the foreign matter. For example, where PVdF is attached
to the surface of the jacket, the PVdF is removed by wiping off the
surface of the jacket with a dimethylformamide solution, followed by
measuring the thickness of the jacket.

[0078] When this film jacket is used, the electrode group is desirably
adhered to the inner surface of the jacket by an adhesive layer formed on
at least a portion of the surface of the electrode group. With this
arrangement, the jacket can be fixed to the surface of the electrode
group. So, it is possible to prevent the nonaqueous electrolyte from
permeating into the boundary between the electrode group and the jacket.

[0079] A thin lithium ion secondary battery as an example of the
nonaqueous electrolyte secondary battery according to the present
invention will now be described below with reference to FIGS. 1 and 2.

[0080] FIG. 1 is a sectional view showing this example of the first
nonaqueous electrolyte secondary battery according to the present
invention. FIG. 2 is an enlarged sectional view showing a portion A in
FIG. 1. FIG. 3 is a schematic view showing the boundaries and their
vicinities of a positive electrode, separator, and negative electrode in
the secondary battery shown in FIG. 1.

[0081] As shown in FIG. 1, a jacket 1 made of, for example, a film
surrounds an electrode group 2. This electrode group 2 has a structure
formed by spirally winding a stack including a positive electrode,
negative electrode, and separator, and compression-molding the coil in
the direction of diameter. As shown in FIG. 2, the stack is formed by
stacking a separator 3; a positive electrode 12 including a positive
electrode layer 4, a positive electrode collector 5, and a positive
electrode layer 4; a separator 3; a negative electrode 13 including a
negative electrode layer 6, a negative electrode collector 7, and a
negative electrode layer 6; a separator 3; a positive electrode 12
including a positive electrode layer 4, a positive electrode collector 5,
and a positive electrode layer 4; a separator 3; and a negative electrode
13 including a negative electrode layer 6, and a negative electrode
collector 7 in this order as seen from the lower side in the drawing. The
negative electrode collector 7 is the outermost layer of the electrode
group 2. An adhesive layer 8 is present on the surface of the electrode
group 2. The adhesive layer 8 is adhered to the inner surface of the
jacket 1. As shown in FIG. 3, in voids of the positive electrode layer 4,
the separator 3, and the negative electrode layer 6, an adhesive polymer
9 is held, respectively. The positive electrode 12 and the separator 3
are adhered to each other by adhesive polymers 9 that are dispersedly
present in the positive electrode layer 4 and the separator 3 and in a
boundary between the positive electrode layer 4 and the separator 3. The
negative electrode 13 and the separator 3 are adhered to each other by
adhesive polymers 9 that are dispersedly present in the negative
electrode layer 6 and the separator 3 and in a boundary between the
negative electrode layer 6 and the separator 3. The electrode group 2 in
the jacket 1 is impregnated with a nonaqueous electrolyte. A band-like
positive electrode lead 10 has one end connected to the positive
electrode collector 5 of the electrode group 2 and the other end
extending from the jacket 1. A band-like negative electrode lead 11 has
one end connected to the negative electrode collector 7 of the electrode
group 2 and the other end extending from the jacket 1.

[0082] In FIG. 1, the adhesive layer 8 is formed on the entire surface of
the electrode group 2. However, this adhesive layer 8 can also be formed
only on a portion of the surface of the electrode group 2, though it is
desirable to form the adhesive layer 8 on at least a surface
corresponding to the outermost circumferential surface of the electrode
group. Further, it is possible to omit the adhesive layer 8.

[0083] The nonaqueous electrolyte secondary battery provided with an
electrode group meeting the conditions given in item (a) described
previously can be manufactured by, for example, the manufacturing method
(I) described below. Of course, the manufacturing method of the
nonaqueous electrolyte secondary battery of the present invention is not
limited to the method described below as far as the manufacturing method
falls within the scope defined in the present invention.

<Manufacturing Method (I)>

[0084] (First Step)

[0085] A porous sheet used as a separator is interposed between positive
and negative electrodes to form an electrode group.

[0086] This electrode group is desirably formed by any of: spirally
winding positive and negative electrodes with a separator not containing
an adhesive polymer interposed between them; spirally winding a positive
and negative electrodes with a separator not containing an adhesive
polymer interposed between them and compressing the spiral or coil in the
direction of diameter; and folding positive and negative electrodes a
plurality of times with a separator not containing an adhesive polymer
interposed between them. When the electrode group is formed by any of
these methods, in a second step (to be described later) it is possible to
allow a solution of an adhesive polymer to permeate the positive
electrode, negative electrode, and separator and at the same time prevent
this solution from permeating the whole boundary between the positive
electrode and the separator and the whole boundary between the negative
electrode and the separator. Consequently, the adhesive polymer can be
dispersedly present in the positive electrode, negative electrode, and
separator and can be dispersedly present in a boundary between the
positive electrode and the separator and in a boundary between the
negative electrode and the separator.

[0087] The positive electrode is formed by suspending an active material,
conducting agent, and binder in an appropriate solvent, coating a
collector with this suspension, and drying the collector to form a thin
plate. Examples of the active material, conducting agent, binder, and
collector are materials similar to those enumerated above in the
explanation of 1) Positive electrode.

[0088] The negative electrode is formed by kneading a carbon material
which absorbs and desorbs lithium ions and binder in the presence of a
solvent, coating a collector with the resultant suspension, drying the
collector, and pressing the collector once or 2 to 5 times with desired
pressure. Examples of the carbonaceous material, binder, and collector
are materials similar to those enumerated above in the explanation of 2)
Negative electrode.

[0089] The separator not containing an adhesive polymer is made of, e.g.,
a porous sheet. Examples of the porous sheet is material similar to those
enumerated above in the explanation of 3) Separator.

[0090] (Second Step)

[0091] The electrode group is housed in a bag-like film jacket. In this
case it is desirable that the stacked section of this electrode group be
seen through the opening of the jacket. A solution prepared by dissolving
an adhesive polymer in a solvent is injected into the electrode group in
the jacket to impregnate the electrode group with the solution.

[0092] Examples of the film jacket are materials analogous to those
enumerated above in the explanation of 6) Jacket.

[0093] Examples of the adhesive polymer are materials analogous to those
enumerated above in the explanation of 5) Adhesive polymer. Of these
polymers, polyvinylidene fluoride is most preferred.

[0094] The solvent is desirably an organic solvent having a boiling point
of 200° C. or less. Dimethylformamide (boiling point 153°
C.) is an example of this organic solvent. If the boiling point of the
organic solvent exceeds 200° C., a long drying time may be
necessary when the temperature of drying (to be described later) is set
at 100° C. or less. The lower limit of the organic solvent boiling
point is preferably 50° C. If the organic solvent boiling point is
lower than 50° C., the organic solvent may evaporate while the
solution is injected into the electrode group. The upper limit of the
boiling point is more preferably 180° C., and its lower limit is
more preferably 100 C.

[0095] The concentration of the adhesive polymer in the solution is
preferably 0.05 to 2.5 wt % for the following reasons. If the
concentration is less than 0.05 wt %, it may become difficult to adhere
the positive electrode and the separator, and the negative electrode and
the separator, with sufficient strength. On the other hand, if the
concentration exceeds 2.5 wt %, it may become difficult to obtain enough
porosity to hold the nonaqueous electrolyte in place, and the interface
impedance of the electrode may increase. If the interface impedance
increases, the capacity and the large discharge characteristic greatly
degrade. A more favorable range of the concentration is 0.1 to 1.5 wt %.

[0096] When the concentration of the adhesive polymer in the solution is
0.05 to 2.5 wt %, the injection amount of solution is preferably 0.1 to 2
ml per 100 mAh battery capacity for the following reasons. If the
injection amount is less than 0.1 ml, it may become difficult to well
improve the adhesion strength between the positive electrode and the
separator and between the negative electrode and the separator. On the
other hand, if the injection amount exceeds 2 ml, the lithium ion
conductivity of the secondary battery may decrease, or its internal
resistance may increase. This may make it difficult to improve the
discharge capacity, large discharge characteristic, and charge/discharge
cycle life. A more favorable range of the injection amount is 0.15 to 1
ml per 100 mAh battery capacity.

[0097] (Third Step)

[0098] The electrode group is dried in a vacuum to evaporate the solvent
in the solution. By this step, adhesive polymers are held in voids of the
positive electrode, the negative electrode and the separator, and the
positive electrode and the separator are adhered to each other by
adhesive polymers that are dispersedly present in the positive electrode
and the separator and in a boundary between the positive electrode and
the separator. Simultaneously, the negative electrode and the separator
are adhered to each other by adhesive polymers that are dispersedly
preset in the negative electrode and the separator and in a boundary
between the negative electrode and the separator. By this step, water
removal contained in the electrode group can be simultaneously performed.

[0099] The electrode group can contain a slight amount of solvent.

[0100] The drying is preferably performed at 100° C. or less for
the following reasons. If the drying temperature exceeds 100° C.,
the separator may greatly thermally shrink. If this large thermal
shrinkage occurs, the separator warps, and this makes it difficult to
strongly adhere the positive electrode, negative electrode, and
separator. This thermal shrinkage readily occurs when a porous film
containing polyethylene or polypropylene is used as a separator. The
lower the drying temperature, the more easily this separator thermal
shrinkage can be suppressed. However, if the drying temperature is lower
than 40° C., the solvent may become difficult to well evaporate.
Therefore, the drying temperature is more preferably 40 to 100° C.

[0101] (Fourth Step)

[0102] After a nonaqueous electrolyte is injected into the electrode group
in the jacket, the opening of the jacket is sealed to complete the thin
nonaqueous electrolyte secondary battery unit.

[0103] As the nonaqueous electrolyte, materials similar to those
enumerated in item 4) above in the explanation of the aforementioned
first nonaqueous electrolyte secondary battery can be used.

[0104] In the above manufacturing method, after the electrode group is
housed in the jacket the solution of the adhesive polymer is injected.
However, this solution can also be injected before the electrode group is
housed in the jacket. If this is the case, the electrode group is formed
by interposing the separator between the positive and negative
electrodes. This electrode group is impregnated with the solution and
dried under vacuum to evaporate the solvent in the solution such that the
adhesive polymer is allowed to fill the pores within the positive
electrode, the negative electrode and the separator. After this electrode
group is housed in the jacket, the nonaqueous electrolyte is injected,
and the opening of the jacket is sealed to manufacture the thin
nonaqueous electrolyte secondary battery unit. In this case, it is
possible to use a metal can as the jacket in place of a film. It is
possible to coat the outer circumferential surface of the electrode group
with an adhesive before the electrode group is housed in the film jacket
so as to allow the electrode group to be adhered to the jacket.

[0105] (Fifth Step)

[0106] An initial charging is applied to the battery unit thus prepared
under temperature of 30° C. to 80° C. and under a charging
rate falling within a range of between 0.05 C and 0.5 C. The initial
charging under these conditions can be applied for only one cycle or a
plurality of cycles. It is also possible to store the battery unit before
the initial charging for 1 hour to about 20 hours under the temperature
of 30° C. to 80° C. Incidentally, the charging rate "1 C"
represents the current value required for charging a nominal capacity
(Ah) in an hour.

[0107] It is important to set the temperature range appropriately in
performing the initial charging. Specifically, if the initial charging
temperature is lower than 30° C., the nonaqueous electrolyte has a
high viscosity, making it difficult to have the positive electrode, the
negative electrode and the separator impregnated uniformly with the
nonaqueous electrolyte. As a result, an internal impedance is increased.
Also, the utilization of the active material is lowered. On the other
hand, if the initial charging temperature exceeds 80° C., the
binder contained in the positive and negative electrodes is deteriorated.

[0108] Where the charging rate in the initial charging treatment is set to
fall within a range of between 0.05 and 0.5 C, the rate of expansion of
the positive and negative electrodes caused by the charging can be
retarded appropriately. As a result, the nonaqueous electrolyte is
allowed to permeate uniformly into the positive and negative electrodes.

[0109] Since the initial charging treatment makes it possible to have the
pores of the electrodes and the separator impregnated with the nonaqueous
electrolyte uniformly, it is possible to diminish the internal impedance
of 1 kHz within the nonaqueous electrolyte secondary battery.
Specifically, it is possible to set the product between the battery
capacity and the internal impedance of 1 kHz to fall within a range of
between 10 mΩAh and 110 mΩAh. As a result, the utilization of
the active material can be increased, making it possible to increase the
substantial capacity of the battery. It is also possible to improve the
charge-discharge cycle characteristics and the large discharge
characteristics of the battery.

[0110] Description will now be given to a nonaqueous electrolyte secondary
battery comprising an electrode group meeting the condition (b) described
previously and a nonaqueous solvent containing γ-butyrolactone in
an amount larger than 50% by volume and not larger than 95% by volume
based on the total amount of the nonaqueous solvent.

[0111] In the secondary battery of this type, the positive electrode, the
negative electrode and the separator are made integral by the thermal
curing of the binder contained in the positive and negative electrodes.

[0112] A separator similar to that described previously under the heading
"(3) Separator" is used in the secondary battery of this type. Also, a
jacket similar to that described previously under the heading "(6)
Jacket" is used as a jacket housing the electrode group.

[0113] The positive electrode is constructed such that a positive
electrode layer containing an active material, a binder and a conducting
agent is formed on a single surface or both surfaces of the collector. It
is possible to use an active material, a binder, a conducting agent and a
collector similar to those described previously under the heading "(1)
Positive electrode".

[0114] The negative electrode is constructed such that a negative
electrode layer containing a carbon material for absorbing-desorbing
lithium ions and a binder is formed on a single surface or both surfaces
of a collector. It is possible to use a carbonaceous material, a binder
and a collector similar to those described previously under the heading
"(2) Negative Electrode".

[0115] The negative electrode layer contains a carbon material capable of
absorbing and desorbing lithium ions as described above. Alternatively,
it is possible for the negative electrode layer to contain a metal such
as aluminum, magnesium, tin or silicon, a metal compound selected from a
metal oxide, a metal sulfide and a metal nitride, or a lithium alloy. It
is possible to use a metal oxide, a metal sulfide, a metal nitride and a
lithium alloy similar to those described previously under the heading
"(2) Negative electrode".

[0116] The secondary battery of this type can be manufactured by method
(II) described in the following.

<Manufacturing Method (II)>

[0117] (First Step)

[0118] An electrode group is formed by any of methods (a) to (c) below.

[0119] (a) Positive and negative electrodes are spirally wound with a
separator interposed between them.

[0120] (b) Positive and negative electrodes are spirally wound with a
separator interposed between them, and the coil is compressed in the
direction of diameter.

[0121] (c) Positive and negative electrodes are folded twice or more with
a separator interposed between them.

[0122] (Second Step)

[0123] The electrode group is housed in a bag-like film jacket.

[0124] (Third Step)

[0125] The electrode group is molded while being heated to 40 to
120° C.

[0126] This molding is desirably performed such that the electrode group
is compressed in the direction of diameter if it is formed by method (a),
and is compressed in the direction of stacking if it is formed by method
(b) or (c).

[0127] The molding can be performed by press molding or forcing into a
mold.

[0128] The electrode group is heated when it is molded for the reasons
explained below. In the electrode group, the separator contains no
adhesive polymer. If this electrode group is molded at room temperature,
spring back occurs after the molding, i.e., gaps are formed between the
positive electrode and the separator and between the negative electrode
and the separator. As a result, the contact areas between the positive
electrode and the separator and between the negative electrode and the
separator are decreased, leading to an increased internal impedance. When
the electrode group is molded at 40° C. or higher, the binders
contained in the positive and negative electrodes can be thermally set,
with the result that the hardness of the electrode group can be
increased. Since this suppresses the spring back after the molding, the
contact areas between the positive electrode and the separator and
between the negative electrode and the separator can be increased. Also,
the large contact areas can be maintained even if the charge-discharge
cycles are repeated. On the other hand, if the temperature of the
electrode group exceeds 120° C., the separator may greatly
thermally shrink. The temperature is more preferably 60 to 100° C.

[0129] The molding by heating to a specific temperature described above
can be performed at normal pressure or reduced pressure or in a vacuum.
This heat molding is desirably performed at reduced pressure or in a
vacuum because the efficiency of water removal from the electrode group
can be improved.

[0130] When the molding is performed by press molding, the pressure is
preferably set to between 0.01 and 20 kg/cm2 for the following
reasons. If the pressure is lower than 0.01 kg/cm2, it is difficult
to suppress the spring back amount after the molding step. If the
pressure is higher than 20 kg/cm2, however, the porosity of the
electrode group tends to be lowered. As a result, the electrode group may
fail to retain a sufficiently large amount of the nonaqueous electrolyte.

[0131] (Fourth Step)

[0132] After a nonaqueous electrolyte is injected into the electrode group
in the jacket, the opening of the jacket is sealed to complete the
nonaqueous electrolyte secondary battery unit.

[0133] In the above manufacturing method, after being housed in the
jacket, the electrode group is molded while being heated to a specific
temperature. However, this heat molding can also be performed before the
electrode group is housed in the jacket. If this is the case, an
electrode group is formed in the first step described earlier and molded
while being heated to 40 to 120° C. Then, this electrode group is
housed in the jacket. After that, the nonaqueous electrolyte is injected,
and the opening of the jacket is sealed to complete the nonaqueous
electrolyte secondary battery unit. It is possible to use a metal can as
a jacket in place of a film.

[0134] (Fifth Step)

[0135] An initial charging is applied to the secondary battery unit thus
assembled under temperature of 30° C. to 80° C. and under a
charging rate falling within a range of between 0.05 C and 0.5 C. The
initial charging under these conditions can be applied for only one cycle
or a plurality of cycles. It is also possible to store the battery unit
before the initial charging for 1 hour to about 20 hours under the
temperature of 30° C. to 80° C. The temperature and the
charging rate in the initial charging treatment are similar to those
described previously.

[0136] Since the initial charging treatment makes it possible to have the
pores of the electrodes and the separator impregnated with the nonaqueous
electrolyte uniformly, it is possible to diminish the internal impedance
of 1 kHz within the nonaqueous electrolyte secondary battery.
Specifically, it is possible to set the product between the battery
capacity and the internal impedance of 1 kHz to fall within a range of
between 10 mΩAh and 110 mΩAh. As a result, the utilization of
the active material can be increased, making it possible to increase the
substantial capacity of the battery. It is also possible to improve the
charge-discharge cycle characteristics and the large discharge
characteristics of the battery.

[0137] In the nonaqueous electrolyte secondary battery according to the
first embodiment of the present invention, it is possible to use a metal
can such as an aluminum can as a jacket. In this case, a laminate
structure consisting of the positive electrode, the negative electrode
and the separator is spirally wound and put in the metal can so as to
prepare a desired nonaqueous electrolyte secondary battery. In this case,
the adhesive layer and the adhesive polymer need not be used.

[0138] A nonaqueous electrolyte secondary battery according to a second
embodiment of the present invention will now be described.

[0139] The secondary battery according to the second embodiment of the
present invention comprises:

[0140] an electrode group including a positive electrode having a
collector and a positive electrode layer supported on a single surface or
both surfaces of the collector and containing an active material, a
negative electrode having a collector and a negative electrode layer
supported on a single surface or both surfaces of the collector and
containing a material capable of absorbing and desorbing lithium ions,
and a separator interposed between the positive electrode and the
negative electrode;

[0141] a nonaqueous electrolyte impregnated in the electrode group and
containing a nonaqueous solvent and a lithium salt dissolved in the
nonaqueous solvent; and

[0142] a jacket housing the electrode group.

[0143] The porosity of the positive electrode layer is lower than that of
the negative electrode layer. Also, the thickness of the positive
electrode layer should be 10 to 100 μm. Further, the nonaqueous
solvent should contain γ-butyrolactone in an amount of 40 to 95% by
volume based on the total amount of the nonaqueous solvent.

[0144] In the secondary battery of the present invention, it is possible
for the positive electrode, the negative electrode and the separator not
to be formed as an integral structure. However, it is desirable to form
an integral structure under the conditions given in item (a) or (b):

[0145] (a) The positive electrode and the separator are adhered to each
other by an adhesive polymer present in at least a part of the boundary
therebetween, and the negative electrode and the separator are adhered to
each other by an adhesive polymer present in at least a part of the
boundary therebetween. Particularly, the positive electrode and the
separator preferably be adhered to each other by an adhesive polymer
dotted inside and at the boundary between the positive electrode and the
separator. Also, the negative electrode and the separator preferably be
adhered to each other by an adhesive polymer dotted inside and at the
boundary between the negative electrode and the separator.

[0146] (b) The positive electrode, the negative electrode and the
separator are made integral by thermally curing a binder contained in
each of the positive electrode and the negative electrode.

[0147] The construction in any of items (a) and (b) given above makes it
possible to further suppress the swelling of the jacket.

[0148] It is desirable for the secondary battery of the present invention
to have a product between the battery capacity (Ah) and the battery
internal impedance (mΩ) at 1 kHz of not smaller than 10 mΩAh
and not larger than 110 mΩAh. Incidentally, the battery capacity
denotes a nominal capacity or the discharge capacity at the time when the
battery is discharged at a discharge rate of 0.2 C. More preferably, the
product between the battery capacity and the battery internal impedance
at 1 kHz should fall within a range of between 20 mΩAh and 60
mΩAh.

[0149] The product between the battery capacity and the battery internal
impedance at 1 kHz can be set to fall within a range of between 10
mΩAh and 110 mΩAh by the "Manufacturing method (I)" or
"Manufacturing method (II)" described previously. In the Manufacturing
method (I) noted above, the addition amount and distribution of the
adhesive polymer and the initial charging conditions are set to permit
the product between the battery capacity and the internal impedance
should be set to fall within a range of between 10 mΩAh and 110
mΩAh. In the Manufacturing method (II), the temperature and the
pressure in the pressing step for forming the electrode group and the
initial charging conditions should be set to permit the product between
the battery capacity and the internal impedance to fall within a range of
between 10 mΩAh and 110 mΩAh.

[0150] The nonaqueous electrolyte secondary battery equipped with an
electrode group meeting the conditions given in item (a) above will now
be described.

[0151] 1) Positive Electrode

[0152] This positive electrode is constructed such that a positive
electrode layer containing an active material, a conducting agent, an
adhesive polymer and a binder is formed on a single surface or both
surfaces of a collector. It is possible to use the active material, the
conducting agent, the adhesive polymer and the binder similar to those
described previously.

[0153] The thickness of the positive electrode layer should be 10 to 100
μm. The thickness of the positive electrode layer represents a
distance between the surface of the positive electrode layer facing the
separator and the surface of the positive electrode layer facing the
collector, as shown in, for example, FIG. 4. Specifically, where a
positive electrode layer P is carried on each surface of a collector S,
the thickness T of the positive electrode layer P represents the distance
between a surface P1 of the positive electrode layer P facing the
separator and a surface P2 of the positive electrode layer P facing
the collector S. It follows that, where positive electrode layers are
formed on both surfaces of the collector, one positive electrode layer
has a thickness of 10 to 100 μm. Naturally, the total thickness of the
two positive electrode layers formed on both surfaces of the collector is
20 to 200 μm. If the positive electrode layer is thinner than 10
μm, the weight ratio and the volume ratio of the collector is unduly
increased so as to lower the energy density. The lower limit in the
thickness is preferably 30 μm, and most preferably 50 μm. On the
other hand, if the positive electrode layer is thicker than 100 μm,
the nonaqueous electrolyte is concentrated on the surface of the positive
electrode at rapid change and at rapid discharge. As a result, the
electrode reaction scarcely proceeds inside the positive electrode,
leading to a shortened cycle life. The upper limit in the thickness is
preferably 85 μm, and most preferably 60 μm. It is more desirable
to set the thickness of the positive electrode layer to fall within a
range of between 10 μm and 60 μm. Where the thickness falls within
this range, the large discharge characteristics and the cycle life can be
markedly improved. More preferably, the positive electrode layer should
fall within a range of between 30 μm and 50 μm.

[0154] For measuring the thickness of the positive electrode, 10 optional
points apart from each other by at least 1 cm are selected for actually
measuring the thickness at each point, followed by averaging the measured
values to determine the thickness of the positive electrode. Where the
positive electrode consists of a collector and positive electrode layers
formed on both surfaces of the collector, the thickness of the positive
electrode is measured after one of the positive electrode layers is
removed. Then, the remaining positive electrode layer is removed from the
collector for measuring the thickness of the collector. The thickness of
the collector is measured at 10 optional points apart from each other by
at least 1 cm, and the measured values are averaged to determine the
thickness of the collector. Naturally, the difference in thickness
between the positive electrode and the collector provides the thickness
of the positive electrode layer.

[0155] The porosity of the positive electrode layer is lower than that of
the negative electrode layer. It is desirable to set the porosity of the
positive electrode layer to fall within a range of between 25% and 40%.
If the porosity is lower than 25%, the nonaqueous electrolyte tends to
fail to permeate uniformly into the positive electrode layer even if the
thickness of the particular layer is restricted. On the other hand, if
the porosity exceeds 40%, it is difficult to obtain a high capacity,
i.e., a high energy density. More preferably, the porosity of the
positive electrode layer should fall within a range of between 30% and
35%.

[0156] A porous conductive substrate or a non-porous conductive substrate
can be used as the collector. It is possible to use, for example,
aluminum, stainless steel or nickel for forming the conductive substrate.
The thickness of the collector should desirably be set to fall within a
range of between 5 and 20 μm. If the thickness falls within the
particular range, it is possible to increase the mechanical strength of
the positive electrode while suppressing an increase in the weight of the
positive electrode.

[0157] 2) Negative Electrode

[0158] The negative electrode is constructed such that a negative
electrode layer containing a carbon material capable of absorbing and
desorbing lithium ions, an adhesive polymer and a binder is formed on one
surface or both surfaces of a collector.

[0159] It is possible to use a carbon material capable of absorbing and
desorbing lithium ions, a conducting agent, an adhesive polymer and a
binder similar to those described previously.

[0160] It is desirable for the negative electrode layer to have a
thickness of 10 to 100 μm. The thickness represents the distance
between the surface of the negative electrode layer facing the separator
and the surface of the negative electrode layer facing the collector.
Where negative electrode layers are formed on both surfaces of the
collector, one negative electrode layer has a thickness of 10 to 100
μm. Naturally, the total thickness of the two negative electrode
layers formed on both surfaces of the collector is 20 to 200 μm. If
the negative electrode layer is thinner than 10 μm, the weight ratio
and the volume ratio of the collector are increased, making it difficult
to increase sufficiently the energy density. The lower limit in the
thickness is preferably 30 μm, and most preferably 50 μm. On the
other hand, if the thickness of the negative electrode layer exceeds 100
μm, the nonaqueous electrolyte tends to be concentrated on the surface
of negative electrode, making it difficult to improve sufficiently the
cycle life of the secondary battery. The upper limit in the thickness is
preferably 85 μm, and most preferably 60 μm. More preferably, the
thickness of the negative electrode layer should be set to fall within a
range of between 10 and 60 μm. Where the thickness falls within the
particular range, the large discharge characteristics and the cycle life
can be markedly improved. Most preferably, the negative electrode layer
should fall within a range of between 30 and 50 μm.

[0161] For measuring the thickness of the negative electrode, 10 optional
points apart from each other by at least 1 cm are selected for actually
measuring the thickness at each point, followed by averaging the measured
values to determine the thickness of the negative electrode. Where the
negative electrode consists of a collector and negative electrode layers
formed on both surfaces of the collector, the thickness of the negative
electrode is measured after one of the negative electrode layers is
removed. Then, the remaining negative electrode layer is removed from the
collector for measuring the thickness of the collector. The thickness of
the collector is measured at 10 optional points apart from each other by
at least 1 cm, and the measured values are averaged to determine the
thickness of the collector. Naturally, the difference in thickness
between the negative electrode and the collector provides the thickness
of the negative electrode layer.

[0162] It is desirable to set the porosity of the negative electrode layer
to fall within a range of between 35% and 50%. If the porosity is lower
than 35%, the nonaqueous electrolyte fails to be distributed uniformly,
resulting in possibility of precipitating lithium dendride. On the other
hand, if the porosity exceeds 50%, it is difficult to obtain a high
battery capacity, i.e., a high energy density. More preferably, the
porosity should fall within a range of between 35% and 45%.

[0163] The carbon material should be mixed in an amount of 90 to 98% by
weight. Also, the binder should be mixed in an amount of 2 to 20% by
weight. Particularly, it is desirable for the carbon material to be
contained in an amount of 10 to 70g/cm2 on one surface in the
prepared negative electrode.

[0164] The density of the negative electrode layer should desirably fall
within a range of between 1.20 and 1.50 g/cm3.

[0165] As the collector, a porous conductive substrate or a conductive
substrate having no pores can be used. These conductive substrates can be
made from, for example, copper, stainless steel, or nickel. The thickness
of the collector should desirably fail within a range of between 5 and 20
μm. If the thickness falls within the particular range, it is possible
to ensure a sufficiently high mechanical strength of the negative
electrode while properly suppressing the weight of the negative
electrode.

[0166] The negative electrode layer contains a carbon material capable of
absorbing-desorbing lithium ions as described previously. In addition, it
is possible for the negative electrode layer to contain metals such as
aluminum, magnesium, tin, and silicon, a metal compound selected from the
group consisting of a metal oxide, a metal sulfide, and a metal nitride,
and a lithium alloy. It is possible to use a metal oxide, a metal
sulfide, a metal nitride and a lithium alloy similar to those described
previously in conjunction with the nonaqueous electrolyte secondary
battery according to the first embodiment of the present invention.

[0167] 3) Separator

[0168] This separator is formed of a porous sheet. It is possible to use a
porous sheet similar to that described previously in conjunction with the
nonaqueous electrolyte secondary battery according to the first
embodiment of the present invention.

[0169] 4) Nonaqueous Electrolyte

[0170] The nonaqueous electrolyte used in the present invention is
prepared by dissolving a lithium salt in a mixed nonaqueous solvent
containing γ-butyrolactone (BL) in an amount of 40 to 95% by volume
based on the total amount of the nonaqueous solvent. It is desirable for
the mixed nonaqueous solvent to contain the largest amount of BL. If the
BL amount is smaller than 40% by volume, a gas is likely to be generated
under high temperatures even if the thickness of the positive electrode
layer is defined. Also, where the nonaqueous solvent contains a cyclic
carbonate together with BL, the viscosity of the solvent is markedly
increased because the ratio of the cyclic carbonate is relatively high.
As a result, the electrical conductivity and the permeability of the
nonaqueous electrolyte are markedly lowered. It follows that the
charge-discharge cycle characteristics, the large discharge
characteristics, and the discharge characteristics under low temperatures
about -20° C. are lowered, even if the thickness of the positive
electrode layer is defined. On the other hand if the BL amount exceeds
95% by volume, a reaction takes place between the negative electrode and
BL so as to impair the charge-discharge characteristics. To be more
specific, if the negative electrode containing, e.g., a carbon material
capable of absorbing and desorbing lithium ions, is reacted with BL to
bring about a reducing decomposition of the nonaqueous electrolyte, a
film inhibiting the charge-discharge reaction is formed on the surface of
the negative electrode. As a result, a current concentration tends to
take place in the negative electrode so as to bring about undesirable
phenomena. For example, lithium metal is precipitated on the surface of
the negative electrode. Alternatively, an impedance is increased at the
interface of the negative electrode so as to lower the charge-discharge
efficiency of the negative electrode and to impair the charge-discharge
cycle characteristics. Preferably, the BL content of the nonaqueous
solvent should fall within a range of between 60% and 90% by volume.
Where the BL content of the nonaqueous solvent falls within the range
specified in the present invention, the gas generation during storage of
the secondary battery under high temperatures can be suppressed more
effectively. Also, it is possible to further improve the discharge
capacity under low temperatures about -20° C. More preferably, the
BL content of the nonaqueous solvent should fall within a range of
between 75% by volume and 90% by volume.

[0171] It is desirable to use a cyclic carbonate together with BL in the
present invention because the cyclic carbonate permits improving the
charge-discharge efficiency.

[0172] The cyclic carbonate used in the present invention includes, for
example, propylene carbonate (PC), ethylene carbonate (EC), vinylene
carbonate (VC), and trifluoropropylene carbonate (TFPC). Particularly, if
EC is used together with BL, the charge-discharge characteristics and the
large discharge characteristics can be markedly improved. It is also
desirable to prepare a mixed solvent by mixing BL with at least one kind
of a third solvent selected from the group consisting of PC, VC, TFPC,
diethyl carbonate (DEC), methyl ethyl carbonate (MEC) and an aromatic
compound. The mixed solvent of the particular construction permits
improving the charge-discharge cycle characteristics.

[0173] In order to decrease the viscosity of the mixed solvent, it is
possible for the nonaqueous solvent containing BL to further contain 20%
by volume or less of a solvent having a low viscosity selected from the
group consisting of, for example, a chain carbonate, a chain ether, and a
cyclic ether.

[0174] Preferred combinations of the nonaqueous solvents used in the
present invention include, for example, a combination of BL and EC, a
combination of BL and PC, a combination of BL, EC and DEC, a combination
of BL, EC and MEC, a combination of BL, EC, MEC and VC, a combination of
BL, EC and VC, a combination of BL, PC and VC, and a combination of BL,
EC, PC and VC. In this case, it is desirable to set the mixing ratio of
EC to fall within a range of between 5 and 40% by volume. It should be
noted in this connection that, if the mixing amount of EC is smaller than
5% by volume, it is difficult to cover densely the surface of the
negative electrode with a protective film, giving rise to a possibility
that a reaction may take place between the negative electrode and BL. As
a result, it is difficult to improve sufficiently the charge-discharge
cycle characteristics. On the other hand, if the mixing amount of EC
exceeds 40% by volume, the viscosity of the nonaqueous electrolyte is
unduly increased so as to lower the ionic conductance. As a result, it is
difficult to improve sufficiently the charge-discharge cycle
characteristics, the large discharge characteristics and the low
temperature discharge characteristics. More preferably, the EC amount
should fall within a range of between 10 and 35% by volume. Also, the
solvent consisting of at least one compound selected from the group
consisting of DEC, MEC, PC and VC serves to forms a dense protective film
on the surface of the negative electrode so as to lower the impedance at
the interface of the negative electrode. The addition amount of any of
these solvents is not particularly limited as far as it is possible to
obtain the particular function described above. It should be noted,
however, that, if the amount of at least one solvent selected from the
group consisting of DEC, MEC, PC and VC exceeds 10% by volume, the
viscosity of the nonaqueous electrolyte tends to be increased so as to
lower the ionic conductance. Therefore, it is desirable to use at least
one solvent selected from the group consisting of DEC, MEC, PC and VC in
an amount not exceeding 10% by volume. More preferably, at least one of
these solvents should be used in an amount of 2% by volume or less. On
the other hand, the lower limit in the addition amount of at least one of
these solvents should be 0.001% by volume, preferably, 0.05% by volume.

[0175] Particularly, it is desirable for the nonaqueous solvent to contain
40 to 95% by volume of BL, EC and VC. The nonaqueous electrolyte
secondary battery comprising a nonaqueous electrolyte containing the
nonaqueous solvent noted above and a negative electrode containing a
carbon material capable of absorbing-desorbing lithium ions permits
markedly lowering the impedance at the interface of the negative
electrode and also permits suppressing the metal lithium precipitation on
the negative electrode so as to improve the charge-discharge efficiency
of the negative electrode. As a result, it is possible to suppress the
gas generation during storage of the secondary battery under high
temperatures, thereby preventing a jacket having a thickness of 3 mm or
less from being deformed while realizing an excellent large discharge
characteristics and a long life. It is considered reasonable to
understand that the reasons for such a prominent improvement of the
negative electrode characteristics are as follows. Specifically, in the
secondary battery, a protective film consisting of EC is formed on the
surface of the negative electrode, and a thin and dense film consisting
of VC is further formed on the surface of the negative electrode. It is
considered reasonable to understand that the reaction between BL and the
negative electrode is inhibited, thereby lowering the impedance and
preventing the metal lithium precipitation.

[0176] It is possible to use a nonaqueous solvent containing 40 to 95% by
volume of BL, EC and an aromatic compound in place of the mixed
nonaqueous solvent of the composition described previously. The aromatic
compound is at least one compound selected from the group consisting of
benzene, toluene, xylene, biphenyl and terphenyl. EC is deposited on the
surface of the negative electrode containing, for example, a carbon
material capable of absorbing-desorbing lithium ions so as to form a
protective film, thereby suppressing the reaction between the negative
electrode and BL. In this case, it is desirable for the mixed nonaqueous
solvent to contain 5 to 40% by volume of EC for the reasons described
previously. Preferably, the EC content should fall within a range of
between 10% by volume and 35% by volume. On the other hand, the benzene
ring of the aromatic compound tends to be adsorbed easily on the surface
of the negative electrode containing, for example, a carbon material
capable of absorbing-desorbing lithium ions so as to suppress the
reaction between BL and the negative electrode. As described above, the
nonaqueous electrolyte based on a mixed nonaqueous solvent containing 40
to 95% by volume of BL, EC and an aromatic compound makes it possible to
suppress sufficiently the reaction between the negative electrode and BL
so as to improve the charge-discharge cycle characteristics of the
secondary battery. It is desirable for the mixed nonaqueous solvent to
further contain at least one solvent selected from the group consisting
of DEC, MEC, PC, TFPC and VC. By adding at least one solvent selected
from the group consisting of DEC, MEC, PC, TFPC and VC, the reaction
between the negative electrode and BL can be suppressed more effectively,
leading to a further improvement in the charge-discharge cycle
characteristics. Particularly, it is desirable to use VC as an additional
solvent. The addition amount of a third solvent consisting of at least
one compound selected from the group consisting of an aromatic compound,
DEC, MEC, PC, TFPC and VC is not particularly limited. In other words,
the addition amount can be determined appropriately as far as the
particular function described above can be performed. It should be noted,
however, that, if the mixing ratio of the third solvent exceeds 10% by
volume, it is difficult to suppress sufficiently the decomposition of the
nonaqueous electrolyte by oxidation under high temperatures.
Alternatively, the viscosity of the nonaqueous electrolyte tends to be
increased so as to lower the ionic conductance. Naturally, it is
desirable for the volume ratio of the third solvent in the nonaqueous
solvent to be at most 10% by volume. Preferably, the volume ratio of the
third solvent should be at most 2% by volume. On the other hand, the
lower limit of the volume ratio of the third component should be 0.001%
by volume, preferably 0.05% by volume.

[0177] It is possible to use an electrolytic salt, which is contained in
the nonaqueous electrolyte, similar to that described previously in
conjunction with the nonaqueous electrolyte secondary battery according
to the first embodiment of the present invention. Particularly, it is
desirable to use LiPF6 or LiBF4.

[0178] The amount of the electrolytic salt dissolved in the nonaqueous
solvent should desirably be 0.5 to 2.0 mol/l (liter).

[0179] It is possible to add 0.1 to 1% of a surfactant such as trioctyl
phosphate to the nonaqueous electrolyte in order to improve the
wettability of the nonaqueous electrolyte with the separator.

[0180] It is desirable to set the amount of the nonaqueous electrolyte to
fall within a range of between 0.2 g and 0.6 g per 100 mAh of the battery
unit capacity for the reasons described previously in conjunction with
the nonaqueous electrolyte secondary battery according to the first
embodiment of the present invention. More preferably, the amount of the
nonaqueous electrolyte should fall within a range of between 0.4 g/100
mAh and 0.55 g/100 mAh.

[0181] 5) Adhesive Polymer

[0182] It is desirable for the adhesive polymer to be capable of
maintaining a high adhesivity while holding the nonaqueous electrolyte.
Further, the adhesive polymer should desirably exhibit a high lithium
ionic conductance. To be more specific, it is possible to use adhesive
polymers similar to those described previously in conjunction with
nonaqueous electrolyte secondary battery according to the first
embodiment of the present invention. Particularly, it is desirable to use
polyvinylidene fluoride as the adhesive polymer.

[0183] It is desirable for the adhesive polymer to assume a porous
structure having fine pores within the cavities of the positive
electrode, the negative electrode, and the separator. The adhesive
polymer assuming a porous structure is capable of holding a nonaqueous
electrolyte.

[0184] The total amount of the adhesive polymer contained in the secondary
battery should desirably fall within a range of between 0.1 mg/100 mAh
and 6 mg/100 mAh of the battery capacity. More preferably, the total
amount of the adhesive polymer should be 0.2 to 1 mg/100 mAh of the
battery capacity.

[0185] 6) Jacket

[0186] A first jacket made of a sheet having a thickness of 0.5 mm or less
including a resin layer or a second jacket having a thickness of 0.3 mm
or less is used in the preset invention. The first and second jackets are
as already described in conjunction with the first nonaqueous electrolyte
secondary battery.

[0187] When the film jacket is used, the electrode group is desirably
adhered to the inner surface of the jacket by an adhesive layer formed on
at least a portion of the surface of the electrode group. With this
arrangement, the jacket can be fixed to the surface of the electrode
group. So, it is possible to prevent the nonaqueous electrolyte from
permeating into the boundary between the electrode group and the jacket.

[0188] A thin lithium ion secondary battery as an example of the
nonaqueous electrolyte secondary battery according to the present
invention will now be described below with reference to FIGS. 5 to 7.

[0189] FIG. 5 is a sectional view showing this example of the first
nonaqueous electrolyte secondary battery according to the second
embodiment of the present invention. FIG. 6 is an enlarged sectional view
showing a portion B in FIG. 5. Further, FIG. 7 is a schematic view
showing the boundaries and their vicinities of a positive electrode,
separator, and negative electrode in the secondary battery shown in FIG.
5.

[0190] As shown in FIG. 5, a jacket 21 made of, for example, a film
surrounds an electrode group 22. This electrode group 22 has a structure
formed by spirally winding a stack including a positive electrode,
negative electrode, and separator, and compression-molding the coil in
the direction of diameter. As shown in FIG. 6, the stack is formed by
stacking a separator 23; a positive electrode 32 including a positive
electrode layer 24, a positive electrode collector 25, and a positive
electrode layer 24; a separator 23; a negative electrode 33 including a
negative electrode layer 26, a negative electrode collector 27, and a
negative electrode layer 26; a separator 23; a positive electrode 32
including a positive electrode layer 24, a positive electrode collector
25, and a positive electrode layer 24; a separator 23; and a negative
electrode 33 including a negative electrode layer 26, and a negative
electrode collector 27 in this order as seen from the lower side in the
drawing. The negative electrode collector 27 is the outermost layer of
the electrode group 22. An adhesive layer 28 is present on the surface of
the electrode group 22. The adhesive layer 28 is adhered to the inner
surface of the jacket 21. As shown in FIG. 7, in voids of the positive
electrode layer 24, the separator 23, and the negative electrode layer
26, an adhesive polymer 29 is held, respectively. The positive electrode
32 and the separator 23 are adhered to each other by adhesive polymers 29
that are dispersedly present in the positive electrode layer 24 and the
separator 23 and in a boundary between the positive electrode layer 24
and the separator 23. The negative electrode 33 and the separator 23 are
adhered to each other by adhesive polymers 29 that are dispersedly
present in the negative electrode layer 26 and the separator 23 and in a
boundary between the negative electrode layer 26 and the separator 23.
The electrode group 22 in the jacket 21 is impregnated with a nonaqueous
electrolyte. A band-like positive electrode lead 30 has one end connected
to the positive electrode collector 25 of the electrode group 22 and the
other end extending from the jacket 21. A band-like negative electrode
lead 31 has one end connected to the negative electrode collector 27 of
the electrode group 22 and the other end extending from the jacket 21.

[0191] Incidentally, in the construction shown in FIG. 5, the adhesive
layer 28 is formed on the entire surface of the electrode group 22.
However, it is also possible to form the adhesive layer 28 in a part of
the electrode group 22. Where the adhesive layer 28 is formed in a part
of the electrode group 22, it is desirable to form the adhesive layer 28
on at least a plane corresponding to the outermost circumferential
surface of the electrode group 22. Also, it is possible to omit the
adhesive layer 28.

[0192] The nonaqueous electrolyte secondary battery provided with an
electrode group satisfying the condition of item (a) described previously
can be manufactured by, for example, the manufacturing method (I)
described previously in conjunction with the nonaqueous electrolyte
secondary battery according to the first embodiment of the present
invention. To reiterate, the secondary battery can be manufactured by the
method comprising the step of preparing an electrode group by interposing
a porous sheet used as a separator between the positive electrode and the
negative electrode, the step of impregnating the electrode group with a
solution prepared by dissolving an adhesive polymer in a solvent, the
step of applying a vacuum drying to the electrode group, the step of
impregnating the electrode group with a nonaqueous electrolyte, followed
by sealing the electrode group within the jacket so as to assemble a thin
nonaqueous electrolyte secondary battery unit, and the step of applying
an initial charging at a charging rate of 0.05 C to 0.5 C under a
temperature of 30° C. to 80° C. Of course, the
manufacturing method of the nonaqueous electrolyte secondary battery of
the present invention is not limited to the method described above, as
far as the battery falls within the technical scope of the present
invention.

[0193] A nonaqueous electrolyte secondary battery provided with the
electrode group meeting the condition given in item (b) previously and
with a nonaqueous electrolyte including a nonaqueous solvent containing
40 to 95% by volume of γ-butyrolactone will now be described.

[0194] In this secondary battery, the positive electrode, the negative
electrode and the separator are made integral by thermal curing of the
binder contained in the positive and negative electrodes.

[0195] It is possible to use a separator similar to that described
previously under the heading "3) Separator" in conjunction with the
nonaqueous electrolyte secondary battery according to the second
embodiment of the present invention. On the other hand, it is to use a
jacket housing the electrode group similar to that described previously
under the heading "6) Jacket" in conjunction with the nonaqueous
electrolyte secondary battery according to the second embodiment of the
present invention.

[0196] The positive electrode is constructed such that a positive
electrode layer containing an active material, a binder and a conducting
agent is held on one surface or both surfaces of the collector. It is
possible to use the active material, the binder, conducting agent and the
collector similar to those described previously under the heading "1)
Positive electrode" in conjunction with the nonaqueous electrolyte
secondary battery according to the second embodiment of the present
invention.

[0197] The thickness of the positive electrode layer should be 10 to 100
μm for the reasons described previously. Where the positive electrode
layer is held on both surfaces of the collector, the total thickness of
two positive electrode layers should fall within a range of between 20
μm and 200 μm. The lower limit in the thickness is preferably 30
μm, and most preferably 50 μm. On the other hand, the upper limit
in the thickness is preferably 85 μm, and most preferably 60 μm.
Preferably, the thickness of the positive electrode layer should be 10 to
60 μm for the reasons described previously in conjunction with the
nonaqueous electrolyte secondary battery according to the second
embodiment of the present invention. More preferably, the positive
electrode layer should fall within a range of between 30 μm and 50
μm.

[0198] The porosity of the positive electrode layer should be lower than
that of the negative electrode layer. The porosity of the positive
electrode layer should be set to fall within a range of 25% to 40%, more
preferably between 30% and 35%.

[0199] The negative electrode is constructed such that a negative
electrode layer containing a carbon material capable of absorbing and
desorbing lithium ions and a binder is held on one surface or both
surfaces of a collector. It is to use the carbon material, the binder and
the collector similar to those described previously under the heading "2)
Negative electrode" in conjunction with the nonaqueous electrolyte
secondary battery according to the second embodiment of the present
invention.

[0200] The thickness of the negative electrode layer should be 10 to 100
μm for the reasons described previously. Where the negative electrode
material layer is held on both surfaces of the collector, the total
thickness of two negative electrode layers should fall within a range of
between 20 μm and 200 μm. The lower limit in the thickness is
preferably 30 μm, and most preferably 50 μm. On the other hand, the
upper limit in the thickness is preferably 85 μm, and most preferably
60 μm. Preferably, the thickness of the negative electrode layer
should be 10 to 60 μm for the reasons described previously in
conjunction with the nonaqueous electrolyte secondary battery according
to the second embodiment of the present invention. Most preferably, the
negative electrode layer should fall within a range of between 30 μm
and 50 μm.

[0201] The porosity of the negative electrode active material should
desirably be 35% to 50% for the reasons described previously. More
preferably, the porosity of the negative electrode active material should
desirably be 35% to 45%.

[0202] It is desirable to mix the carbon material in an amount of 90 to
98% by weight and the binder in an amount of 2 to 20% by weight.
Particularly, the amount of the carbon material should desirably be 10 to
70 g/cm2. On the other hand, it is desirable to set the density of
the negative electrode layer to fall within a range of between 1.20 to
1.50 g/cm3.

[0203] The secondary battery can be manufactured by the method described
previously under the heading "Manufacturing method (II)" in conjunction
with the nonaqueous electrolyte secondary battery according to the second
embodiment of the present invention. To reiterate, the secondary battery
can be manufactured by the method comprising the step of preparing an
electrode group by interposing a separator between a positive electrode
and a negative electrode, the step of molding the electrode group while
heating the electrode group to 40 to 120° C., the step of
impregnating the electrode group with a nonaqueous electrolyte, followed
by sealing the electrode group within the jacket so as to assemble a
nonaqueous electrolyte secondary battery (battery unit), and the step of
applying an initial charging treatment to the battery unit at a charging
rate of 0.05 to 0.5 C under temperature of 30° C. to 80° C.

[0204] Incidentally, in the nonaqueous electrolyte secondary battery
according to the second embodiment of the present invention, it is
possible to use, for example, an aluminum can as a jacket and to house
the electrode group consisting of a positive electrode, a negative
electrode and a separator within the aluminum can. In this case, it is
unnecessary to use an adhesive layer or an adhesive polymer.

[0205] As described above in detail, the nonaqueous electrolyte secondary
battery according to the present invention comprises an electrode group
including a positive electrode, a negative electrode containing a
material capable of absorbing and desorbing lithium ions, and a separator
interposed between the positive and negative electrodes, a nonaqueous
electrolyte impregnated in the electrode group and prepared by dissolving
a lithium salt in a nonaqueous solvent, and a jacket for housing the
electrode group, said jacket having a thickness of 0.3 mm or less. It
should be noted that the nonaqueous solvent used in the present invention
should contain γ-butyrolactone in an amount larger than 50% by
volume and not larger than 95% by volume based on the total amount of the
nonaqueous solvent.

[0206] In a nonaqueous electrolyte secondary battery, the thickness of the
secondary battery is required to be as small as 3 to 4 mm. It is
undesirable to decrease the thickness of the electrode group in an
attempt to decrease the thickness of the secondary battery because the
battery capacity is lowered if the electrode group is made unduly thin.
In order to make the secondary battery sufficiently thin without
impairing the battery capacity, it is necessary to decrease the thickness
of the jacket. However, if the thickness of the jacket is not larger than
0.3 mm, the jacket is deformed by the gas generated during storage of the
secondary battery under high temperatures. Under the circumstances, it
was difficult in the past to use a jacket having less than 0.3 mm of
thickness and, thus, it was unavoidable to sacrifice the battery capacity
in making the secondary battery sufficiently thin.

[0207] It should be noted that γ-butyrolactone is excellent in its
chemical stability. Therefore, a nonaqueous solvent containing a
predetermined amount of γ-butyrolactone serves to suppress the
reaction between the positive electrode active material and the
nonaqueous electrolyte when the secondary battery is stored under high
temperatures, with the result that the decomposition of the nonaqueous
electrolyte by oxidation is suppressed. It follows that the amount of the
generated gas can be suppressed to a low level, making it possible to
prevent the thin jacket having a thickness of 0.3 mm or less from being
swollen. Therefore, it is possible to decrease the thickness of the
secondary battery while maintaining a practical large discharge
characteristics and charge-discharge cycle characteristics and while
scarcely sacrificing the battery capacity. It follows that it is possible
to obtain a thin nonaqueous electrolyte secondary battery excellent in
the large discharge characteristics, having a long life, and high in both
the weight energy density and the volume energy density.

[0208] It is possible to prevent the reaction between the negative
electrode and γ-butyrolactone so as to suppress decomposition of
the nonaqueous electrolyte by reduction by setting the charging
temperature at 30 to 80° C. and the charging rate at 0.05 to 0.5 C
in applying an initial charging to the secondary battery. As a result,
the impedance at the interface of the negative electrode can be lowered
and the metal lithium precipitation can be suppressed. It follows that it
is possible to improve the large discharge characteristics and the
charge-discharge cycle characteristics of the secondary battery.

[0209] In the secondary battery of the present invention, the
concentration of the lithium salt contained in the nonaqueous electrolyte
is set at 0.5 mol/l or more so as to improve the ionic conductance of the
nonaqueous electrolyte. Therefore, the large discharge characteristics
and the cycle life can be further improved.

[0210] In the secondary battery of the present invention, a carbon
material capable of absorbing and desorbing lithium ions preferably be
contained in the negative electrode. This case, if the nonaqueous solvent
further contains ethylene carbonate, a protective film is formed on the
surface of the negative electrode. This makes it also possible to further
suppress the reaction between γ-butyrolactone and the negative
electrode so as to improve the large discharge characteristics and the
cycle life. It is possible to allow the nonaqueous solvent to contain at
least one third solvent selected from the group consisting of vinylene
carbonate, propylene carbonate, diethyl carbonate, methyl ethyl
carbonate, trifluoropropylene and an aromatic compound. In this case, the
surface of the negative electrode is densely covered with a protective
film so as to markedly suppress the reaction between the negative
electrode and γ-butyrolactone. As a result, the large discharge
characteristics and the cycle life can be further improved.

[0211] In the secondary battery of the present invention, the product
between the cell capacity (Ah) and the internal impedance (mΩ) of
the battery at 1 kHz is set to fall within a range of between 10 mAh and
110 mAh, making it possible to further improve the large discharge
characteristics and the cycle life.

[0212] In the secondary battery of the present invention, the separator
contains a porous sheet having an air permeability of 600 sec/100
cm3 or less. Use of the particular separator makes it possible to
allow the nonaqueous electrolyte containing γ-butyrolactone to
permeate uniformly into the separator. As a result, the ionic conductance
of the separator can be improved so as to improve the large discharge
characteristics and the cycle life.

[0213] It should be noted that the jacket having a thickness of 0.3 mm or
less, which is included in the secondary battery of the present
invention, tends to follow the expansion and shrinkage of the electrode
group accompanying the charging and discharging operations. As a result,
the jacket fails to hold the electrode group strongly. Therefore, with
progress in the charge-discharge cycle, the contact areas between the
positive electrode and the separator and between the negative electrode
and the separator tend to be diminished. Such being the situation, an
adhesive polymer is used in at least a part of the boundaries between the
positive electrode and the separator and between the negative electrode
and the separator. In this case, the positive electrode, the negative
electrode and the separator can be held together by the adhesive polymer
in spite of the progress in the charge-discharge cycle. As a result, it
is possible to suppress elevation of the battery internal impedance so as
to further improve the cycle life. At the same time, the gas generation
can be further suppressed during storage of the secondary battery under
high temperatures.

[0214] In the secondary battery of the present invention, an adhesive
polymer is dotted inside the positive electrode and the separator and at
the interface between the positive electrode and the separator to allow
these positive electrode and the separator to form an integral structure.
Likewise, an adhesive polymer is dotted inside the negative electrode and
the separator and at the interface between the negative electrode and the
separator to allow these negative electrode and the separator to form an
integral structure. It follows that it is possible to improve the bonding
strength between the positive electrode and the separator and the bonding
strength between the negative electrode and the separator while
suppressing the internal resistance at a low level. As a result,
elevation of the internal impedance can-be suppressed so as to improve
the cycle life. At the same time, the gas generation can be suppressed
more effectively during storage of the secondary battery under high
temperatures.

[0215] In the secondary battery of the present invention, the positive
electrode, the negative electrode and the separator can be allowed to
have an integral structure by the thermal curing of the binder contained
in the positive and negative electrodes, making it possible to increase
the contact areas between the positive electrode and the separator and
between the negative electrode and the separator. In addition, the large
contact areas can be maintained even if the charge-discharge cycles are
repeated. As a result, elevation of the internal impedance can be
suppressed so as to further improve the cycle life. At the same time, the
gas generation can be suppressed during storage of the secondary battery
under high temperatures.

[0216] The present invention also provides a method of manufacturing a
nonaqueous electrolyte secondary battery, comprising an electrode group
including a positive electrode, a negative electrode containing a
material capable of absorbing and desorbing lithium ions, and a separator
interposed between the positive electrode and the negative electrode, and
a nonaqueous electrolyte impregnated in the electrode group and including
a nonaqueous solvent and a lithium salt dissolved in the nonaqueous
solvent. The manufacturing method of the present invention is featured in
that the nonaqueous solvent contains 55 to 95% by volume of
γ-butyrolactone based on the total amount of the nonaqueous
solvent, and an initial charging is performed at a charging rate of 0.05
to 0.5 C under temperatures of 30° C. and 80° C.

[0217] According to the method of the present invention for manufacturing
a nonaqueous electrolyte secondary battery, it is possible to permit the
nonaqueous electrolyte to permeate sufficiently into the positive and
negative electrodes and the separator. As a result, the internal
impedance of the secondary battery can be diminished, and the utilization
of the active material can be increased. It follows that the substantial
battery capacity can be improved.

[0218] The nonaqueous electrolyte secondary battery of the present
invention comprises an electrode group including a positive electrode, a
negative electrode containing a material capable of absorbing-desorbing
lithium ions, and a separator interposed between the positive electrode
and the negative electrode; a nonaqueous electrolyte impregnated in the
electrode group and comprising a nonaqueous solvent and a lithium salt
dissolved in the nonaqueous solvent; and a jacket housing the electrode
group and consisting of a sheet having a thickness of 0.5 mm or less
including a resin layer. The nonaqueous solvent contains
γ-butyrolactone in an amount larger than 50% by volume and not
larger than 95% by volume of the entire nonaqueous solvent.

[0219] The secondary battery of the particular construction permits
suppressing the gas generation and, thus, prevents the jacket consisting
of a sheet having a thickness of 0.5 mm or less including a resin layer
from being swollen. It follows that it is possible to use a jacket light
in weight. In addition, it is possible to maintain large discharge
characteristics and charge-discharge cycle characteristics sufficiently
satisfactory for the practical use of the battery. It follows that the
present invention provides a nonaqueous electrolyte secondary battery
excellent in large discharge characteristics, having a long life, and
exhibiting a high weight energy density.

[0220] The nonaqueous electrolyte secondary battery according to the
present invention comprises an electrode group including a collector and
a positive electrode layer containing an active material and formed on
one surface or both surfaces of the collector, a negative electrode
including a collector and a negative electrode layer containing a
material capable of absorbing and desorbing lithium ions and formed on
one surface or both surfaces of the collector, and a separator interposed
between the positive electrode and the negative electrode, a nonaqueous
electrolyte including a nonaqueous solvent and a lithium salt dissolved
in the nonaqueous solvent, and a jacket having a thickness of 0.3 mm or
less and housing the electrode group. The porosity of the positive
electrode layer is lower than that of the negative electrode layer. Also,
the thickness of the positive electrode layer is 10 to 100 μm.
Further, the nonaqueous solvent contains 40 to 95% by volume of
γ-butyrolactone based on the total amount of the nonaqueous
solvent.

[0221] If the nonaqueous electrolyte impregnated in the negative electrode
is not distributed uniformly in the nonaqueous electrolyte secondary
battery, current concentration takes place in the negative electrode so
as to cause precipitation of lithium dendride. To avoid this difficulty,
the porosity is set high in the negative electrode so as to improve the
permeability of the nonaqueous electrolyte. On the other hand, such a
problem does not take place in the positive electrode. In addition, if
the porosity of the positive electrode is set at a high level as in the
negative electrode, the density of the positive electrode active material
layer is lowered, resulting in failure to obtain a high battery capacity.
Such being the situation, the porosity of the positive electrode layer is
set lower than that of the negative electrode layer.

[0222] A nonaqueous electrolyte containing γ-butyrolactone tends to
fail to be permeated uniformly into the electrode such as the positive
electrode or the negative electrode. If it is intended to allow the
nonaqueous electrolyte to permeate into the positive electrode having a
low porosity, the nonaqueous electrolyte permeates into only the surface
region of the electrode, leading to a marked shortening of the cycle
life.

[0223] The permeability of the electrolyte into the positive electrode
layer can be improved by setting the thickness of the positive electrode
layer having a low porosity at 10 to 100 μm as in the present
invention. As a result, the nonaqueous electrolyte containing
γ-butyrolactone can be permeated uniformly both into the positive
electrode layer and the negative electrode layer. As a result,
γ-butyrolactone is enabled to exhibit its excellent resistance to
oxidation. The improved permeability makes it possible for the nonaqueous
solvent to contain a broaden range of BL, i.e., 40 to 95% by volume of
γ-butyrolactone, while allowing the nonaqueous electrolyte
containing the particular nonaqueous solvent to serve to suppress the gas
generation during storage of the secondary battery under high
temperatures. As a result, it is possible to prevent the thin jacket
having a thickness of 0.3 mm or less from being swollen. Therefore, it is
possible to decrease the thickness of the secondary battery while
maintaining a practical large discharge characteristics and
charge-discharge cycle characteristics and while scarcely sacrificing the
battery capacity. It follows that it is possible to obtain a thin
nonaqueous electrolyte secondary battery excellent in the large discharge
characteristics, having a long life, and high in both the weight energy
density and the volume energy density.

[0224] It is possible to prevent the reaction between the negative
electrode and γ-butyrolactone so as to suppress decomposition of
the nonaqueous electrolyte by reduction by setting the charging
temperature at 30 to 80° C. and the charging rate at 0.05 to 0.5 C
in applying an initial charging to the secondary battery. As a result,
the impedance at the interface of the negative electrode can be lowered
and the metal lithium precipitation can be suppressed. It follows that it
is possible to improve the large discharge characteristics and the
charge-discharge cycle characteristics of the secondary battery.

[0225] In the secondary battery of the present invention, the thickness of
the negative electrode layer is set at 10 to 100 μm so as to improve
the electrolyte permeability into the negative electrode layer.
Therefore, it is possible to suppress the gas generation during storage
of the secondary battery under high temperatures and to improve the large
discharge characteristics and the charge-discharge cycle life.

[0226] In the secondary battery of the present invention, the
concentration of the lithium salt in the nonaqueous electrolyte is set at
0.5 mol/l or more so as to improve the ionic conductance of the
nonaqueous electrolyte and, thus, to improve the large discharge
characteristics and the cycle life.

[0227] In the secondary battery of the present invention, the negative
electrode preferably contain a carbon material capable of absorbing and
desorbing lithium ions. In this case, if the nonaqueous solvent further
contains ethylene carbonate, a protective film is formed on the surface
of the negative electrode. It follows that the reaction between the
negative electrode and γ-butyrolactone can be further suppressed so
as to further improve the large discharge characteristics and the cycle
life. It is also possible for nonaqueous solvent used in the present
invention to further contain at least one kind of third solvent selected
from the group consisting of vinylene carbonate, propylene carbonate,
diethyl carbonate, methyl ethyl carbonate, trifluoropropylene and an
aromatic compound. In this case, the surface of the negative electrode is
densely covered with a protective film so as to markedly suppress the
reaction between the negative electrode and γ-butyrolactone. As a
result, the large discharge characteristics and the cycle life can be
further improved.

[0228] In the secondary battery of the present invention, the separator
contains a porous sheet having an air permeability of 600 sec/100
cm3 or less. Use of the particular separator makes it possible to
allow the nonaqueous electrolyte containing γ-butyrolactone to
permeate uniformly into the separator. As a result, the ionic conductance
of the separator can be improved so as to improve the large discharge
characteristics and the cycle life.

[0229] In the secondary battery of the present invention, the positive
electrode and the separator are adhered to each other by an adhesive
polymer present in at least a part of the boundary between the positive
electrode and the separator. Likewise, the negative electrode and the
separator are adhered to each other by an adhesive polymer present in at
least a part of the boundary between the negative electrode and the
separator. As a result, elevation of an internal impedance accompanying
the progress of the charge-discharge cycles can be suppressed so as to
improve the cycle life. At the same time, the gas generation during
storage of the secondary battery under high temperatures can be further
suppressed.

[0230] In the secondary battery of the present invention, an adhesive
polymer is dotted inside the positive electrode and the separator and at
the interface between the positive electrode and the separator to allow
these positive electrode and the separator to form an integral structure.
Likewise, an, adhesive polymer is dotted inside the negative electrode
and the separator and at the interface between the negative electrode and
the separator to allow these negative electrode and the separator to form
an integral structure. It follows that it is possible to improve the
bonding strength between the positive electrode and the separator and the
bonding strength between the negative electrode and the separator while
suppressing the internal resistance at a low level. As a result,
elevation of the internal impedance can be suppressed so as to improve
the cycle life. At the same time, the gas generation can be suppressed
more effectively during storage of the secondary battery under high
temperatures.

[0231] In the secondary battery of the present invention, the positive
electrode, the negative electrode and the separator can be allowed to
have an integral structure by the thermal curing of the binder contained
in the positive and negative electrodes. As a result, the internal
impedance in the initial period of the charge-discharge cycle can be
suppressed and the internal impedance can be maintained in spite of the
progress of the charge-discharge cycles so as to further improve the
cycle life. At the same time, the gas generation can be suppressed during
storage of the secondary battery under high temperatures.

[0232] The nonaqueous electrolyte secondary battery according to the
present invention comprises an electrode group including a collector and
a positive electrode layer containing an active material and formed on
one surface or both surfaces of the collector, a negative electrode
including a collector and a negative electrode layer containing a
material capable of absorbing and desorbing lithium ions and formed on
one surface or both surfaces of the collector, and a separator interposed
between the positive electrode and the negative electrode, a nonaqueous
electrolyte including a nonaqueous solvent and a lithium salt dissolved
in the nonaqueous solvent, and a jacket housing the electrode group. The
jacket is made of a sheet having a thickness of 0.5 mm or less including
a resin layer. The porosity of the positive electrode layer is lower than
that of the negative electrode layer. Also, the thickness of the positive
electrode layer is 10 to 100 μm. Further, the nonaqueous solvent
contains 40 to 95% by volume of γ-butyrolactone based on the total
amount of the nonaqueous solvent.

[0233] The secondary battery of the particular construction permits
suppressing the gas generation and, thus, prevents the jacket made of a
sheet having a thickness of 0.5 mm or less including a resin layer from
being swollen. It follows that it is possible to use a jacket light in
weight. In addition, it is possible to maintain large discharge
characteristics and charge-discharge cycle characteristics sufficiently
satisfactory for the practical use of the battery. It follows that the
present invention provides a nonaqueous electrolyte secondary battery
excellent in large discharge characteristics, having a long life, and
exhibiting a high weight energy density.

[0234] Preferred Examples of the present invention will now be described
in detail.

Example 1

<Manufacture of Positive Electrode>

[0235] First, 91 wt % of a lithium cobalt oxide (LixCoO2:
0≦X≦1) powder, 3.5 wt % of acetylene black, 3.5 wt % of
graphite, 2 wt % of an ethylenepropylenediene monomer (EPDM) powder as a
binder, and toluene were mixed. The two surfaces of a collector made of a
porous aluminum foil (15 μm thick) having 0.5 mm diameter pores at a
ratio of 10 pores per 10 cm2 were coated with the resultant mixture.
This collector was pressed to manufacture a positive electrode having an
electrode density of 3 g/cm3 in which positive electrode layers were
carried by the two surfaces of the collector.

<Manufacture of Negative Electrode>

[0236] 93 wt % of a powder of mesophase pitch-based carbon fibers
heat-treated at 3,000° C. (fiber diameter=8 μm, average fiber
length=20 μm, and average interplanar spacing (d002)=0.3360 nm)
as a carbon material and 7 wt % of polyvinylidene fluoride (PVdF) as a
binder were mixed. A collector made of a porous copper foil (15 μm
thick) having 0.5 mm diameter pores at a ratio of 10 pores per 10
cm2 was coated with the resultant mixture. This collector was dried
and pressed to manufacture a negative electrode having an electrode
density of 1.3 g/cm3 in which negative electrode layer were carried
by the surface of the collector.

<Separator>

[0237] A separator made of a porous polyethylene film which had a
thickness of 25 μm, a heat shrinkage of 20% upon being left to stand
at 120° C. for 1 hr, and a porosity of 50% was prepared.

[0239] A band-like positive electrode lead was welded to the collector of
the positive electrode, and a band-like negative electrode lead was
welded to the collector of the negative electrode. Subsequently, a
laminate structure consisting of the positive electrode, the separator
and the negative electrode laminated in the order mentioned was spirally
wound, followed by flattening the spiral structure to obtain a flat
electrode group.

[0240] A 100 μm thick laminate film formed by covering the two surfaces
of an aluminum foil with polypropylene was molded into a bag shape. The
electrode group was housed in this bag such that the stacked section was
seen through the opening of the bag. 0.3 wt % of polyvinylidene fluoride
(PVdF) as an adhesive polymer was dissolved in dimethylformamide (boiling
point=153° C.) as an organic solvent. The resultant solution was
injected into the electrode group in the laminate film such that the
amount per 100 mAh battery capacity was 0.2 ml. In this manner, the
solution was allowed to penetrate into the electrode group and adhere to
the entire surfaces of the electrode group.

[0241] Next, the electrode group in the laminate film was vacuum-dried at
80° C. for 12 hr to evaporate the organic solvent.

[0242] In this electrode group, the adhesive polymer was held in each of
the void of the positive electrode, the void of negative electrode, and
the void of separator. A porous adhesive layer was formed on the surfaces
of the electrode group. The total amount of PVdF per 100 mAh battery
capacity was 0.6 mg. Note that the total amount of adhesive polymer per
100 mAh battery capacity was calculated from an increase in the weight
from that of the electrode group before impregnating the electrode group
with the adhesive polymer solution.

[0243] The nonaqueous electrolyte was injected into the electrode group in
the laminate film such that the amount per 1 Ah battery capacity was 4.7
g (0.47 g per 100 mAh), thereby assembling a thin nonaqueous electrolyte
secondary battery (battery unit) 3 mm thick, 40 mm wide, and 70 mm high,
having the structure shown in FIGS. 1 and 2 described earlier.

[0244] An initial charging treatment was applied to the nonaqueous
electrolyte secondary battery (battery unit) thus prepared, as follows.
In the first step, the battery unit was left to stand under such a high
temperature as 40° C. for 5 hours, followed by charging the
battery unit under a charging rate of 0.2 C (120 mA) for 10 hours. As a
result, the battery voltage was increased to 4.2V. The charging was
performed under a constant current and a constant voltage. Then, the
battery unit was discharged at a rate of 0.2 C to a battery voltage of
2.7V. Further, a second cycle of the charging was performed under the
conditions similar to those of the initial charging (first cycle) so as
to obtain a thin nonaqueous electrolyte secondary battery.

[0245] The capacity of the nonaqueous electrolyte secondary battery was
measured, and the internal impedance (mΩ) of the battery at 1 kHz
was measured. In order to examine the large discharge characteristics of
the nonaqueous electrolyte secondary battery at room temperature
(20° C.), the capacity retention rate in the discharge step at 2 C
was measured. Also, in order to examine the charge-discharge cycle
characteristics, the secondary battery was charged at 0.5 C for 3 hours
under a constant current and a constant voltage to obtain a battery
voltage of 4.2V, followed by discharge at 1 C to lower the battery
voltage to 2.7V. The charge-discharge cycle described above was repeated
to measure the capacity retention rate after 300 charge-discharge cycles.
Also, after charging to 4.2V of the battery voltage, the battery was
stored at 85° C. for 120 hours so as to measure a swelling of the
battery after the storage. Table 1 shows the electrolyte composition,
initial charging conditions, and the battery characteristics.

Examples 2 to 7

[0246] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the composition of the solvent for the electrolyte
was changed as shown in Table 1, so as to evaluate the battery. The
electrolyte composition, the initial charging conditions, and the battery
characteristics for each of these Examples are shown in Table 1.

Example 8

[0247] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that an aluminum can having a thickness of 0.2 mm was
used as the jacket, an adhesive polymer was not used, and that the
secondary battery was sized at 3.2 mm in thickness, 40 mm in width and 70
mm in height, so as to evaluate the secondary battery. The electrolyte
composition, the initial charging conditions, and the battery
characteristics for Example 8 are also shown in Table 1.

Examples 9 to 11

[0248] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the temperature in the initial charging step was
changed as shown in Table 1, so as to evaluate the battery. The
electrolyte composition, the initial charging conditions, and the battery
characteristics for each of these Examples are shown in Table 1.

Example 12 to 13

[0249] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the composition of the solvent for the electrolyte
was changed as shown in Table 1, so as to evaluate the battery. The
electrolyte composition, the initial charging conditions, and the battery
characteristics for each of these Examples are shown in Table 1.

Comparative Example 1

[0250] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in 100% of BL so as to evaluate the
battery. The electrolyte composition, the initial charging conditions,
and the battery characteristics for Comparative Example 1 are shown in
Table 2.

Comparative Example 2

[0251] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed solvent consisting of BL,
EC and MEC mixed at a volume ratio of 50:25:25 so as to evaluate the
battery. The electrolyte composition, the initial charging conditions,
and the battery characteristics for Comparative Example 2 are also shown
in Table 2.

Comparative Example 3

[0252] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 1.5 mol/l of LiBF4 was dissolved in a mixed
solvent consisting of BL and EC mixed at a volume ratio of 50:50 for
preparing the nonaqueous electrolyte used so as to evaluate the battery.
The electrolyte composition, the initial charging conditions, and the
battery characteristics for Comparative Example 3 are also shown in Table
2.

Comparative Example 4

[0253] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 1 mol/l of LiPF6 was dissolved in a mixed
solvent consisting of BL and MEC mixed at a volume ratio of 25:75 for
preparing the nonaqueous electrolyte used so as to evaluate the battery.
The electrolyte composition, the initial charging conditions, and the
battery characteristics for Comparative Example 4 are also shown in Table
2.

Comparative Example 5

[0254] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 1.5 mol/l of LiBF4 was dissolved in a mixed
solvent consisting of BL and EC mixed at a volume ratio of 25:75 for
preparing the nonaqueous electrolyte used so as to evaluate the battery.
The electrolyte composition, the initial charging conditions, and the
battery characteristics for Comparative Example 5 are also shown in Table
2.

Comparative Example 6

[0255] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 0.8 mol/l of LiPF6 was dissolved in a mixed
solvent consisting of BL and EC mixed at a volume ratio of 50:50 for
preparing the nonaqueous electrolyte used, and that the initial charging
was carried out at 25° C. so as to evaluate the battery. The
electrolyte composition, the initial charging conditions, and the battery
characteristics for Comparative Example 6 are also shown in Table 2.

Comparative Example 7

[0256] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 8, except that 1.5 mol/l of LiBF4 was dissolved in a mixed
solvent consisting of BL and EC mixed at a volume ratio of 50:50 for
preparing the nonaqueous electrolyte used, and that the initial charging
was carried out at 25° C. so as to evaluate the battery. The
electrolyte composition, the initial charging conditions, and the battery
characteristics for Comparative Example 7 are also shown in Table 2.

Comparative Example 8

[0257] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 1.5 mol/l of LiBF4 was dissolved in a mixed
solvent consisting of BL and EC mixed at a volume ratio of 99:1 for
preparing the nonaqueous electrolyte used, and that the initial charging
was carried out at 25° C. so as to evaluate the battery. The
electrolyte composition, the initial charging conditions, and the battery
characteristics for Comparative Example 8 are also shown in Table 2.

Comparative Example 9

[0258] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 1.5 mol/l of LiBF4 was dissolved in a mixed
solvent consisting of BL, EC and DEC mixed at a volume ratio of 50:25:25
for preparing the nonaqueous electrolyte used so as to evaluate the
battery. The electrolyte composition, the initial charging conditions,
and the battery characteristics for Comparative Example 9 are also shown
in Table 2.

Comparative Example 10

[0259] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 1.5 mol/l of LiBF4 was dissolved in a mixed
solvent consisting of EL, EC and MEC mixed at a volume ratio of 50:25:25
for preparing the nonaqueous electrolyte used so as to evaluate the
battery. The electrolyte composition, the initial charging conditions,
and the battery characteristics for Comparative Example 10 are also shown
in Table 2.

Comparative Example 11

[0260] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that 1.5 mol/l of LiBF4 was dissolved in a mixed
solvent consisting of BL, PC and EC mixed at a volume ratio of 50:25:25
for preparing the nonaqueous electrolyte used so as to evaluate the
battery. The electrolyte composition, the initial charging conditions,
and the battery characteristics for Comparative Example 11 are also shown
in Table 2.

[0261] As apparent from Tables 1 and 2, the nonaqueous electrolyte
secondary battery according to any of Examples 1 to 13 comprising a
nonaqueous electrolyte prepared by using a nonaqueous solvent containing
BL in an amount larger than 50% by volume and not larger than 95% by
volume makes it possible to prevent the jacket from being swollen during
storage of the secondary battery under high temperatures and to improve
the discharge capacity at 2 C and the capacity retention rate after 300
charge-discharge cycles.

[0262] On the other hand, the secondary battery according to any of
Comparative Examples 1 to 3 is certainly capable of suppressing the
swelling of the jacket during storage of the secondary battery under high
temperatures. However, the secondary batteries of these Comparative
Examples were found to be inferior to the secondary batteries of Examples
1 to 13 of the present invention in the discharge capacity at 2 C and in
the capacity retention rate after 300 charge-discharge cycles. Also, the
secondary batteries of Comparative Examples 4 to 11 were found to be
inferior to the secondary batteries of Examples 1 to 13 of the present
invention in the swelling of the jacket during storage of the secondary
battery under high temperatures. Incidentally, the nonaqueous electrolyte
contained in the secondary battery of Comparative Example 1 corresponds
to that disclosed in Japanese Patent Disclosure No. 11-97062 referred to
previously. Also, the nonaqueous electrolyte contained in the secondary
battery of Comparative Example 11 corresponds to that disclosed in
Japanese Patent Disclosure No. 4-14769 referred to previously.

Example 14

[0263] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that an aluminum can having a thickness of 0.35 mm was
used as a jacket and that thickness of the electrode group was decreased
to allow the prepared secondary battery to be sized as in Example 1,
i.e., 3 mm in thickness, 40 mm in width, and 70 mm in height. The
secondary battery for Example 14 was 0.4 Ah.

Example 15

[0264] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 24% by volume of ethylene carbonate (EC), 75% by volume of
γ-butyrolactone (BL) and 1% by volume of vinylene carbonate (VC).

Example 16

[0265] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 23% by volume of ethylene carbonate (EC), 75% by volume of
γ-butyrolactone (BL) and 2% by volume of vinylene carbonate (VC).

Example 17

[0266] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 24.5% by volume of ethylene carbonate (EC), 75% by volume
of γ-butyrolactone (BL) and 0.5% by volume of vinylene carbonate
(VC).

Example 18

[0267] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that the nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 25% by volume of ethylene carbonate (EC), 74% by volume of
γ-butyrolactone (BL) and 1% by volume of toluene.

[0268] Each of the secondary batteries obtained in Examples 15 to 18 was
tested as in Example 1 for the battery capacity, the internal impedance,
the capacity retention rate after the discharge at 2 C, the capacity
retention rate after 300 charge-discharge cycles, and the swelling after
storage of the secondary battery at 85° C. Table 3 shows the
results.

[0269] As apparent from Table 3, the secondary battery obtained in each of
Examples 15 to 17 provided with a nonaqueous electrolyte containing BL in
an amount larger than 50% by volume and not larger than 95% by volume, EC
and VC was found to be superior to the secondary battery obtained in
Example 1 in the capacity retention rate after 300 charge-discharge
cycles. On the other hand, the secondary battery obtained in Example 18
provided with a nonaqueous electrolyte containing BL in an amount larger
than 50% by volume and not larger than 95% by volume, EC and aromatic
compound was found to be superior to the secondary battery obtained in
Example 1 in the capacity retention rate after 300 charge-discharge
cycles.

Example 19

[0270] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 1, except that a porous polyethylene film having a thickness of
25 μm, a thermal shrinkage of 20% upon being left to stand at
120° C. for an hour, an air permeability of 90 sec/100 cm3,
and a porosity of 50% was used as the separator.

Example 20

[0271] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 19, except that the air permeability of the porous polyethylene
film used as the separator was set at 580 sec/100 cm3.

Example 21

[0272] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 19, except that the air permeability of the porous polyethylene
film used as the separator was set at 400 sec/100 cm3.

Example 22

[0273] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 19, except that the air permeability of the porous polyethylene
film used as the separator was set at 150 sec/100 cm3.

[0274] Each of the secondary batteries obtained in Examples 19 to 22 was
tested as in Example 1 for the battery capacity, the internal impedance,
the capacity retention rate after the discharge at 2 C, the capacity
retention rate after 300 charge-discharge cycles, and the swelling after
storage of the secondary battery at 85° C. Table 4 shows the
results.

[0275] An electrode group was prepared as in Example 1, except that an
adhesive polymer was not added for preparation of the electrode group.
The electrode group thus prepared was housed in an aluminum can having a
thickness of 0.2 mm, which was used as a jacket. Then, the jacket was
pressed under a pressure of 10 kg/cm2 in a thickness direction of
the electrode group under a high temperature vacuum of 80° C. so
as to thermally cure the binder contained in each of the positive
electrode and the negative electrode, thereby forming an integral
structure.

[0276] On the other hand, a nonaqueous electrolyte was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 24.5% by volume of ethylene carbonate (EC), 75% by volume
of r-butyrolactone (BL), and 0.5% by volume of vinylene carbonate (VC).
The nonaqueous electrolyte thus prepared was poured into the aluminum can
(jacket) such that the electrolyte was permeated into the electrode group
at a rate of 4.7 g per 1 Ah of the battery capacity, followed by sealing
the opening of the aluminum can, thereby obtaining a nonaqueous
electrolyte secondary battery sized at 3.2 mm in thickness, 40 mm in
width and 70 mm in height.

Example 24

[0277] An electrode group was prepared as in Example 1, except that an
adhesive polymer was not added for preparation of the electrode group.
The electrode group thus prepared was housed in an aluminum can having a
thickness of 0.2 mm, which was used as a jacket. Then, the jacket was
pressed under a pressure of 10 kg/cm2 in a thickness direction of
the electrode group under a high temperature vacuum of 80° C. so
as to thermally cure the binder contained in each of the positive
electrode and the negative electrode, thereby forming an integral
structure.

[0278] On the other hand, a nonaqueous electrolyte was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 23% by volume of ethylene carbonate (EC), 75% by volume of
r-butyrolactone (BL), and 2% by volume of vinylene carbonate (VC). The
nonaqueous electrolyte thus prepared was poured into the aluminum can
(jacket) such that the electrolyte was permeated into the electrode group
at a rate of 4.7 g per 1 Ah of the battery capacity, followed by sealing
the opening of the aluminum can, thereby obtaining a nonaqueous
electrolyte secondary battery sized at 3.2 mm in thickness, 40 mm in
width and 70 mm in height.

[0279] Each of the secondary batteries obtained in Examples 23 to 24 was
tested as in Example 1 for the battery capacity, the internal impedance,
the capacity retention rate after the discharge at 2 C, the capacity
retention rate after 300 charge-discharge cycles, and the swelling after
storage of the secondary battery at 85° C. Table 5 shows the
results.

[0280] As apparent from Table 5, each of the secondary batteries obtained
in Examples 23 and 24 exhibited a high battery capacity, was high in the
capacity retention rate in each of the 2 C discharging step and after 300
charge-discharge cycles, and was low in the swelling after storage of the
secondary battery at 85'C.

Example 25

[0281] A nonaqueous secondary battery was prepared as in Example 1, except
that used was a laminate film having a thickness of 500 μm, said
laminate film consisting of an aluminum foil and polypropylene films
formed on both surfaces of the aluminum foil, and that the battery was
sized at 4 mm in thickness, 100 mm in width and 280 mm in height.

[0282] The secondary battery thus prepared was tested as in Example 1 for
the capacity, the capacity retention rate during discharge at 2 C, the
capacity retention rate after 300 charge-discharge cycles, and the
swelling after storage at 85° C. The battery was found to have a
capacity of 6 Ah, a capacity retention rate during discharge at 2 C of
85%, a capacity retention rate after 300 charge-discharge cycles of 90%,
and a swelling after storage at 85° C. of 3%. These experimental
data clearly support that, in the case of using a nonaqueous solvent
containing BL in an amount larger than 50% by volume and not larger than
95% by volume, it is possible to use a laminate film having a thickness
of 0.5 mm as a jacket of a large battery used in, for example, an
electric car.

Example 26

<Preparation of Positive Electrode>

[0283] In the first step, 91% by weight of LixCoO2
(0≦x≦1) powder, 2.5% by weight of acetylene black, 3% by
weight of graphite and 4% by weight of polyvinylidene fluoride (PVdF)
were added to a N-methyl pyrrolidone (NMP) to prepare a slurry, followed
by coating the both surfaces of a collector consisting of an aluminum
foil having a thickness of 10 μm with the resultant slurry and
subsequently drying the coating. Then, the coated collector was pressed
to prepare a positive electrode having a positive electrode layer held on
each surface of the collector. The positive electrode layer had a density
of 3.3 g/cm3, a porosity of 34% and a thickness of 48 μm.

<Preparation of Negative Electrode>

[0284] In the first step, 93% by weight of a carbon material, i.e., a
mesophase pitch based carbon fiber subjected to a heat treatment at
3000° C. and having a fiber diameter of 8 μm, an average fiber
length of 20 μm, and an average interplanar spacing (d002) of
0.3360 nm, and 7% by weight of polyvinylidene fluoride (PVdF) used as a
binder were added to N-methyl pyrrolidone to prepare a slurry. Then, the
both surfaces of a collector consisting of a copper foil having a
thickness of 10 μm were coated with the resultant slurry, followed by
drying the coating. Then, the coated collector was pressed to prepare a
negative electrode having a negative electrode layer held on each surface
of the collector. The negative electrode layer had a density of 1.3
g/cm3, a porosity of 41% and a thickness of 45 μm.

<Separator>

[0285] A porous polyethylene film having a thickness of 20 μm and a
porosity of 50% was used as a separator.

<Preparation of Electrode Group>

[0286] A laminate structure consisting of the positive electrode, the
separator and the negative electrode laminated in the order mentioned was
spirally wound, followed by flattening the spiral structure to obtain a
flat electrode group having a thickness of 2.5 mm, a width of 30 mm and a
height of 50 mm.

<Preparation of Nonaqueous Electrolyte>

[0287] A nonaqueous electrolyte was prepared by dissolving 1.5 mol/l of
LiBF4 in a mixed solvent consisting of ethylene carbonate (EC) and
γ-butyrolactone (BL) mixed at a volume ratio of 25:75.

[0288] Then, a laminate film having a thickness of 100 μm and prepared
by allowing the each surface of an aluminum foil to be covered with a
polypropylene film. The laminate film thus prepared was shaped into a bag
and the electrode group prepared as described above was housed in the
bag. Under this condition, the both surfaces of the bag was nipped by a
holder such that the thickness of the nipped bag was 2.7 mm. On the other
hand, 0.3% by weight of an adhesive polymer of polyvinylidene fluoride
(PVdF) was dissolved in an organic solvent of dimethyl formamide having a
boiling point of 153° C. The resultant solution was poured into
the electrode group housed in the laminate film at a rate of 0.6 ml per
100 mAh of the battery capacity. As a result, the solution permeated into
the electrode group and was attached to the entire surface of the
electrode group.

[0289] In the next step, a vacuum drying was applied at 80° C. to
the electrode group housed in the laminate film for 12 hours so as to
evaporate the organic solvents and to allow the pores of the positive
electrode, the negative electrode and the separator to hold the adhesive
polymer. At the same time, a porous adhesive layer was formed on the
surface of the electrode group.

[0290] Further, the nonaqueous electrolyte was poured into the electrode
group within the laminate film in an amount of 2 g per 1 Ah of the
battery capacity so as to obtain a nonaqueous electrolyte secondary
battery (battery unit) having a thickness of 3 mm, a width of 32 mm and a
height of 55 mm.

[0291] An initial charging treatment was applied to the nonaqueous
electrolyte secondary battery (battery unit) thus prepared, as follows.
In the first step, the battery unit was left to stand under such a high
temperature as 40° C. for 5 hours, followed by charging the
battery unit under a charging rate of 0.2 C (120 mA) for 10 hours. As a
result, the battery voltage was increased to 4.2V. The charging was
performed under a constant current and a constant voltage. Then, the
battery unit was discharged at a rate of 0.2 C to a battery voltage of
2.7V. Further, a second cycle of the charging was performed under the
conditions similar to those of the initial charging (first cycle) so as
to obtain a nonaqueous electrolyte secondary battery.

[0292] In order to examine the large discharge characteristics of the
nonaqueous electrolyte secondary battery at room temperature (20°
C.), the capacity retention rate in the discharge step at 3 C was
measured. In this case, the discharge capacity at 0.2 C was used as the
reference capacity. Also, in order to examine the charge-discharge cycle
characteristics, the secondary battery was charged at 0.7 C for 3 hours
under a constant current and a constant voltage to obtain a battery
voltage of 4.2V, followed by discharge at 1 C to lower the battery
voltage to 2.7V. The charge-discharge cycle described above was repeated
to measure the capacity retention rate after 300 charge-discharge cycles.
Also, after charging to 4.2V of the battery voltage, the battery was
stored at 85° C. for 120 hours so as to measure a swelling of the
battery after the storage. Table 6 shows the initial capacity of the
battery, the thickness of the electrode layer, the amount of
γ-butyrolactone contained in the solvent of the electrolyte, and
the battery characteristics.

Examples 27 to 37

Examples A, B and Comparative Examples 12 to 13

[0293] A thin nonaqueous electrolyte secondary battery was prepared as in
Example 26, except that the thickness of one layer of each of the
positive electrode layer and the negative electrode layer, and the amount
of γ-butyrolactone contained in the solvent of the electrolyte were
changed as shown in Table 6. Table 6 also shows the initial capacity of
the battery, the thickness of the electrode layer, the amount of
γ-butyrolactone contained in the solvent of the electrolyte, and
the battery characteristics.

[0294] To reiterate, the secondary battery for each of Examples 26 to 37,
Examples A and B comprises a positive electrode having a positive
electrode layer having a thickness of 10 to 100 μm, a jacket having a
thickness of 0.3 mm or less, and a nonaqueous solvent containing 40 to
95% by weight of BL. As apparent from Table 6, the secondary battery for
each of these Examples is capable of suppressing the swelling of the
jacket during storage under high temperatures and permits improving the
initial capacity, the discharge capacity at 3 C and the capacity
retention rate after 300 charge-discharge cycles.

[0295] On the other hand, the secondary battery for Comparative Example 12
was found to be inferior to the secondary battery for each of the
Examples of the present invention in the discharge capacity at 3 C and in
the capacity retention rate after 300 charge-discharge cycles. Also, the
secondary battery for Comparative Example 13 was found to be inferior in
the discharge capacity at 3 C than that for the secondary battery for the
Examples of the present invention.

Example 38

[0296] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 26, except that a nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 24.9% by volume of ethylene carbonate (EC), 75% by volume
of γ-butyrolactone, and 0.1% by volume of vinylene carbonate (VC).

Example 39

[0297] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 26, except that a nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 24% by volume of ethylene carbonate (EC), 75% by volume of
γ-butyrolactone, and 1% by volume of vinylene carbonate (VC).

Example 40

[0298] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 26, except that a nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 20% by volume of ethylene carbonate (EC), 75% by volume of
γ-butyrolactone, and 5% by volume of vinylene carbonate (VC).

Example 41

[0299] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 26, except that a nonaqueous electrolyte used was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 25% by volume of ethylene carbonate (EC), 74% by volume of
γ-butyrolactone, arid 1% by volume of toluene.

[0300] The secondary battery obtained in each of Examples 38 to 41 was
tested for the battery capacity, the capacity retention rate during
discharge at 3 C, the capacity retention rate after 300 charge-discharge
cycles, and the swelling after storage at 85° C., as in Example
26. Table 7 shows the results.

[0301] As apparent from Table 7, the secondary battery for each of
Examples 38 to 40 comprising a nonaqueous electrolyte containing 40 to
95% by volume of BL, EC and VC was found to be superior to the secondary
battery for Example 26 in the capacity retention rate after 300
charge-discharge cycles. On the other hand, the secondary battery for
Example 41 comprising a nonaqueous electrolyte containing 40 to 95% by
volume of BL, EC and aromatic compound was found to be superior to the
secondary battery for Example 26 in the capacity retention rate after 300
charge-discharge cycles.

Example 42

[0302] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 26, except that a porous polyethylene film having a thickness of
25 μm, a thermal shrinkage of 20% upon being left to stand at
120° C. for an hour, an air permeability of 90 sec/100 cm3,
and a porosity of 50% was used as a separator.

Example 43

[0303] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 42, except that a porous polyethylene film having an air
permeability of 580 sec/100 cm3 was used as a separator.

Example 44

[0304] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 42, except that a porous polyethylene film having an air
permeability of 400 sec/100 cm3 was used as a separator.

Example 45

[0305] A thin nonaqueous electrolyte secondary battery was obtained as in
Example 42, except that a porous polyethylene film having an air
permeability of 150 sec/100 cm3 was used as a separator.

[0306] The secondary battery obtained in each of Examples 42 to 45 was
tested for the battery capacity, the capacity retention rate during
discharge at 3 C, the capacity retention rate after 300 charge-discharge
cycles, and the swelling after storage at 85° C., as in Example
26. Table 8 shows the results.

[0307] An electrode group was prepared as in Example 26, except that an
adhesive polymer was not added for preparation of the electrode group.
The resultant electrode group was housed in an aluminum can having a
thickness of 0.18 mm, which was used as a jacket. Then, the jacket was
pressed under pressure of 10 kg/cm2 in a thickness direction of the
electrode group under a high temperature vacuum atmosphere of 80°
C. so as to thermally cure the binder contained in the positive and
negative electrode and, thus, to form an integral structure consisting of
the positive electrode, the negative electrode and the separator.

[0308] On the other hand, a nonaqueous electrolyte was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 24.5% of ethylene carbonate (EC), 75% by volume of
γ-butyrolactone and 0.5% by volume of vinylene carbonate (VC). The
nonaqueous electrolyte thus prepared was poured into the electrode group
in an amount of 4.7 g per 1 Ah of the battery capacity, followed by
sealing the opening of the jacket, thereby obtaining a thin nonaqueous
electrolyte secondary battery having a thickness of 3 mm, a width of 32
mm and a height of 55 mm.

Example 47

[0309] An electrode group was prepared as in Example 26, except that an
adhesive polymer was not added for preparation of the electrode group.
The resultant electrode group was housed in an aluminum can having a
thickness of 0.25 mm, which was used as a jacket. Then, the jacket was
pressed under pressure of 10 kg/cm2 in a thickness direction of the
electrode group under a high temperature vacuum atmosphere of 80°
C. so as to thermally cure the binder contained in the positive and
negative electrode and, thus, to form an integral structure consisting of
the positive electrode, the negative electrode and the separator.

[0310] On the other hand, a nonaqueous electrolyte was prepared by
dissolving 1.5 mol/l of LiBF4 in a mixed nonaqueous solvent
consisting of 24% of ethylene carbonate (EC), 75% by volume of
γ-butyrolactone and 2% by volume of vinylene carbonate (VC). The
nonaqueous electrolyte thus prepared was poured into the electrode group
in an amount of 4.7 g per 1 Ah of the battery capacity, followed by
sealing the opening of the jacket, thereby obtaining a thin nonaqueous
electrolyte secondary battery having a thickness of 3 mm, a width of 32
mm and a height of 55 mm.

[0311] The secondary battery obtained in each of Examples 46 to 47 was
tested for the battery capacity, the capacity retention rate during
discharge at 3 C, the capacity retention rate after 300 charge-discharge
cycles, and the swelling after storage at 85° C., as in Example
26. Table 9 shows the results.

[0312] As apparent from Table 9, the secondary battery for each of
Examples 46 and 47 was found to have a high battery capacity, a high
capacity retention rate during discharge at 3 C, a high capacity
retention rate after 300 charge-discharge cycles, and to be capable of
suppressing the swelling during storage at 85° C.

Example 48

[0313] A nonaqueous secondary battery was prepared as in Example 26,
except that used was a laminate film having a thickness of 500 μm,
said laminate film consisting of an aluminum foil and polypropylene films
formed on both surfaces of the aluminum foil, and that the battery was
sized at 4 mm in thickness, 80 mm in width and 220 mm in height.

[0314] The secondary battery thus prepared was tested as in Example 26 for
the capacity, the capacity retention rate during discharge at 3 C, the
capacity retention rate after 300 charge-discharge cycles, and the
swelling after storage at 85° C. The battery was found to have a
capacity of 3.2 Ah, a capacity retention rate during discharge at 3 C of
96%, a capacity retention rate after 300 charge-discharge cycles of 90%,
and a swelling after storage at 85° C. of 3%. These experimental
data clearly support that, in the case where the thickness of the
positive electrode layer is set at 10 to 100 μm and where the
nonaqueous solvent contains 40 to 95% by volume of BL, it is possible to
use a laminate film having a thickness of 0.5 mm as a jacket of a large
battery used in, for example, an electric car.

[0315] As described above in detail, the present invention provides a
nonaqueous electrolyte secondary battery that permits suppressing the
deformation of the jacket during storage under high temperatures and also
permits improving the weight energy density, the volume energy density,
the large discharge characteristics, and the charge-discharge cycle
characteristics.

[0316] Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects is
not limited to the specific details and representative embodiments shown
and described herein. Accordingly, various modifications may be made
without departing from the spirit or scope of the general inventive
concept as defined by the appended claims and their equivalents.